General recommendations

In many households in the developed world the largest sources of CO2 emissions are transportation (car fuel), electricity, heating and food. This means that the average household can reduce its carbon footprint most effectively by taking measures such as:

installing PV panels to generate electricity

buying an electric vehicle (assuming you can charge it with renewable energy)

take public transport more often

switching to a meatless diet

insulate your home

Indirect CO2 emisisons from stuff you buy may also have a big impact, so it's always a good idea to reduce your consumption where possible, and reuse things as much as possible.

Personalised recommendations

Personalised recommendations are rather difficult. There can be very big differences in the CO2 emissions of the average household per country (see for example this list), but it could also be that you've already taken measures and that you are far from being an 'average household'. Consequently it's best to carefully investigate your own lifestyle yourself to see where improvements can be made.

A good way to start is to check a few online carbon footprint calculators to find out how much CO2 emissions you are currently responsible for. Carbon footprint calculators are far from perfect (see also this article), but it's probably the best we have for personalized recommendations. There is a short list of calculators in this site's carbon footprint tag information, but there are also many others. Some calculators, like the Cool California calculator for example, show intermediate results for different categories. This way you can see what currently is your biggest contribution to CO2 emissions, and by tweaking with the parameters you can try to find out what reductions may be possible for you.

To get the most accurate result I recommend selecting calculators that were developed in your country of residence, or that at least allow you to select the country where you live (and hope they use recent and accurate data to make the calculations). I also recommend checking the results of more than 1 calculator, unless you are quite positive the calculator is founded on scientifically sound principles and uses recent and relevant data. Note that not all calculators include indirect CO2 emissions (that were emitted to manufacture a product or service before you started using it), so be careful when comparing results between several calculators.

Alternatively if you really want to make an effort and love spreadsheets and tweaking parameters, I recommend using the GHG emissions spreadsheet created by the EPA.

Tree planting

AFAIK there are no carbon footprint calculators that also show how much you can reduce your CO2 footprint by participating in a tree planting scheme. The efficiency of such a scheme would depend on many things such as; the type of tree planted, the geographical location where they are planted, the amount of space a tree has to grow, and the lifespan of the tree.

A figure you encounter often is that a tree can absorp up to 48 lbs of CO2 per year, but note that this figure is a maximum so the average CO2 absorption during it's lifespan will be lower. To give a rough estimate: a tree will generally absorb around 1 tonne (about 2200 lbs) of CO2 during it's life-time. However, the actual reduction of CO2 will be lower than this calculated amount because the CO2 will be released again once the tree dies. Tree planting alone will never be enough to compensate for all the CO2 that is emitted, so ultimately other solutions are needed. If you do decide to participate in a tree planting scheme, be careful that the particular scheme is legit and doesn't introduce monocultures because this may reduce biodiversity.