After going through the process myself, I've found certain things that work for me and certain things that don't. That is to say, to start with, you should decide which techniques appeal to you the most after looking into it. Some people will say that bottom-up is the only way to go, others say that top-down works best overall, but I used a pretty decent mix of both and filled in the blanks from there.

The first step I suggest you take is to find a blank notebook, blog, Obsidian Portal account, or something similar to organize your work. It will be radically convenient later on when you need to start connecting little things together (X nation was founded when Y nation ended and Z race migrated there, etc.). Cross-referencing your notes becomes important after a while, and the more organized you start, the better off you'll be in the long run. This was something I learned the hard way, after years of having ideas and not recording them and then forgetting them or worse, improving them and forgetting the changes later on. Even after I started writing things down, I've had to re-organize a couple of times due to laziness on my part. Not a fun thing to do with a 50-page word document, heh.

You'll need a map eventually, so why not make it the first thing you do? You don't even need a world map unless you're going extremely top-down, a single continent or even a single region will do. Even most officially published campaign worlds focus on a single continent or part of one, and it seems to work well enough for them.

As for making the map itself, there is a lot of trial and error involved. Some fundamental research on real-world geography will go a long way towards helping you. As quirthanon said, the World Builder's Guidebook is very useful in this regard. I referenced that book countless times in my own work. Finding a copy (digital or otherwise) is highly recommended.

Form an idea of the types of terrain you want to feature, from forests, to plains, deserts to swamps and so on, and figure out how these types of terrain interact with one another. Research will be your friend here and really throughout the entire process, so familiarize yourself with ways of finding information as well.

The medium(s) you use to create the map will depend heavily on how much time and effort you want to spend on it. Map creating software exists but can be pricey and often requires huge investment of time to learn the program and use it, but the trade-off is they produce maps of decent to fantastic visual quality. Using an image editing program such as MS paint or Photoshop has many of the same pros and cons. Old-fashioned pen-and-paper drawing is cheap, easy and can be fun, but unless you have some artistic ability may not live up to your ideals. Again, this step majorly involves trying different things and finding which you feel comfortable with. There really is no wrong way to go about drawing the map as long as you (and eventually your players!) are satisfied with the end result. Personally, I use the old 2nd ed. AD&D Core Rules Program which has a built-in mapping program for overland, city and dungeon maps, and has a very simple "point and click" interface. It also includes a very early version of the Campaign Cartographer software, but unlike the other program, has a steep learning curve.

I realize this post is a wall of text but maybe someone can get some use out of it. I can continue on with other aspects of world building if someone would like, otherwise I'll crawl back into my wizard tower and hope someone finds this useful!