There are many, many fishing knots that you can use for fly fishing. All of them have good uses and functions. There are only four that you as the novice fly angler really need to know to get started in flyfishing.
These are four knots that are easy to tie and the first two are the ones you will most often, like 90% of the time your fishing.
1 > To attach your fly to the tippet > use the the Improved Clinched Knot
2 > To attach new tippet to the leader > use the Surgeons Knots or Blood Knot
3 > To attach your leader to the fly line Loop > use the Perfection Loop
A note on Blood Knot vs Surgeon's Knot. They are both good knows and as a guide for 11 years and just fishing for about 50, I have played with myself and have heard the continuing arguement over using the Surgeons Knot over the Blood Knot. Quite honestly for connecting tippit to leader I have never really seen a big advantage of one over the other, except the Surgeon is just faster to tie for beginners and easier to learn.