The best knots to learn if you are going to get serious about this are the blood knot, for making leaders. The perfection loop, for the same reason. The trilene knot for attaching flies. The non-slip loop knot for the same reason, just to get more wiggle in your fly.
All but the Trilene knot can be learned here: animated knots
The trilene knot can be learned here: trilene knot
If you don't have a factory loop on your fly line you will either want to put on a braided loop, or you will want to use a short piece of heavier mono with a perfection loop on the end and nail knot it to the end of your fly line. The nail knot can be learned in the first link I provided.
Fly fishing is not as hard as it looks. Don't let it intimidate you. It is all just for fun. Welcome to the forum.