This is NOT a strategy post about how to build or play decks. It is a post about how to best use you time and financial resources (gems/coins/sparks) in the game to get the most out of it.

Starting Out.

When you first start playing PvZ you should follow the tutorials and head for the single player campaign to get comfortable with the gameplay.

Make sure to purchase that first 100 gems pack, otherwise you won't "unlock" the multipacks that get you bonus packs when you buy them.

About the basic cards.

The basic cards that you get in PvZ for hero coins are essentially the "starting package", similar to those base cards you get when starting out in games like Hearthstone. The difference is that instead of getting them all at the beginning (though you do get a bunch whenever you unlock each of the initial 6 heroes) or after completing some hero specific quest, you get them in random packs of 3 at a time. This is a very cool and satisfying way of distributing starter cards because it means that thought the period that you are learning the game, you are constantly receiving new cards and have a chance to see how they affect your decks.

These cards cannot be deconstructed for extra sparks (the resource you use to build new cards) and you don't get repeats beyond the 4th card. Once you have them all you will stop receiving those hero coins. It's important not to consider hero coins and basic cards as part of the collectible aspect of the game, they are just the starter package.

Also, do not discount them just because they are basic cards that you get by default. While some of them are just fillers (those 1/1s for 1 brain/sun), a lot of them are high level competitive staples that decks in the highest leagues are using. You will learn about which ones are best as you learn more about the gameplay.

About the Single player campaign.

The single player campaign is a great way to learn the gameplay and, once you are familiar with it, to finish those quests that might be a bit risky to do in ranked play or that you just want to get done quickly. When you are still receiving hero coins, completing a single player game will reward you with these coins, but once you have collected them all, you get nothing from single player games. You also can't play any single player games that you already passed, so once you play through them all, the only one you can play is the very last one. Both of these things are annoying and unsatisfying, but maybe the developers will change this in the future if they get enough feedback (if you dislike this, please write a forum post with your opinion to get the devs to notice). In the meantime, make sure to ONLY use this game mode to learn the game and once you are familiar with it, to finish of hard quests. Preferably, use the ranked and unranked games. If they do change it in the future, you may still be able to get whatever rewards they might add if you don't complete them before.

Playing Multiplayer.

There are two modes of multiplayer: ranked and unranked. In ranked, you get stars every time you win and after a certain number of stars, you go up to the next rank, gaining a small gems reward in the process. If you go up enough ranks you will move onto a different league, gaining a large gems reward. In general, you should be playing ranked since that gives you the most rewards and only playing unranked once you get to a league where losing actually makes you lose stars. You will normally be matched agains someone in a similar rank to yours but occasionally you will be ranked way up or way down. Don't get too upset by this since this is a necessary evil due to there not being enough players to have consistent matching yet.

Collecting gems and purchasing packs.

At first you will get lots of gems due to the initial quests being quite generous and having a bunch of hero quests to go along with them. I recommend using your gems in the following way:

Always save up gems until you reach 2000 gems, and buy the 20+3 multipack. This pack gets you a nice bonus and is not too difficult to achieve, specially during the first few weeks.

If you plan to spent money on the game, purchase the starter pack, which gets you a guaranteed extra hero, and then save your money until you see a flash sale for a discount on getting gems. Use your money then.

You only get that discount once. After that, you will want to save your money for the special events when they sell cards that can only be bought for cash during those periods.

If you have extra budget, consider getting those large starter packs with guaranteed legendaries and guaranteed heroes. Only if you have a nice, large extra budge though.

If you have a smaller budget, your best option is to get as close to 5000 gems as you can, then use your budget to top up the missing gems to get the 50+10 pack.

During the first few weeks, expect to have a very high influx of cards as you will be getting hero coins (use these immediately as soon as you reach 100 to buy basic packs) and lots of gems from the ranked ladder rewards (wait till you reach 2K to buy a multipack). You will also be getting cards and packs from hero quests and if you are lucky, you will get more heroes from the multipacks you buy.

Once you run out hero coins and you reach the higher leagues, your card influx and gem intake will be greatly reduced. At this point you should have bought at least 2 of the 20+3 multipacks and should have plenty of cards to make very competitive decks.

If you are still missing a lot of uncommons to have a set of 4 of each uncommon, then you should continue to save up for the 20+3 multipack. However, once you have most of them, stop saving for these and start saving to purchase the 750 gem packs that have a guaranteed legendary in it. Those packs change every week. Given that the odds of getting heroes from normal packs are fairly low, getting the guaranteed hero is a much better deal. The hero that appears on sale is random so it will take several weeks to get them all. Note that when you already have the hero that is on sale, you will instead see a 300 gem pack with specific cards. I recommend avoiding these unless you are quite certain that the guaranteed cards will really improve your existing decks.

About Hero Quests.

Each hero has 10 different quests that revolve around their base strategies. Some of these quests give you 10-25 gems, others will give you sparks (to craft cards) and others will give you specific cards. There are 2 quests for each hero that give you a specific card, one uncommon and one rare and the quests require you to craft your first copy of that card and then you get another copy as a reward. It is important to know that if you already have 4 of the card that you would get, the quest is automatically achieved. For the gold cards, this is a considerable savings in sparks, so if you have 3 copies of the card in question, I recommend just leaving the quest unfinished until you get that last copy (you are only blocking a couple of quests at this point). This is also why it's best to first fill out your collection of cards, specially the uncommons, then start trying to get the individual heroes.

About the higher up leagues.

Once you get past gold league, climbing up the ranks becomes much, much harder due to the difficulty of the matches, the fact that you lose stars when you lose and that after taco league you need 20 stars to advance. If you reached this far, you are a pretty great player so give yourself a pat on the back! However, at this point the game is mainly designed for hard core players who are playing more for fun and the joy of the game than to accumulate gems. This is where you have to decide if you are playing this game because you enjoy it or because you just want to get more gems. You should also realize an important thing: while there may be decks with lots of legendary cards, most veterans agree that a solid deck foundation is based on uncommons and rare cards, with a few key super-rare cards, so most people should be able to compete quite well agains any deck.

That's about it. It will likely take a few months to reach the highest level of competition, depending on how often you play, but my recommendation is to follow the quest frequency and only play beyond that if you are just really enjoying it. Avoid "grinding", playing for the sake of playing to try to get more gems, as that will just disappoint you.