It is easier than you might think to prune your tomato plants. First you need to figure out what to take off. Look for the juncture between branches and main stems. If you see new growth sticking out in the middle of the V, that is what you want to prune off.
It is amazing how fast they can grow, so check every day or couple of days to see what has popped out. Even if you've missed one and it has grown really big, you can still prune it off.