I have been snowboarding for seven years and for some reason Im very cautious, and rarely catch much air due to some hidden fear inside of me. On the other hand, I have been biking for less than a year, and just yesterday I pulled off almost a 5 foot wheelie drop with good form. I wanted to start getting into the freeride world so first I needed to know how to wheelie. I still cant wheelie for more than 5 seconds, but I can do it long enough, and straight enough to hit a drop in good form.
What you should do is learn to wheelie if you dont know how. Once you get comfortable with that go find some curbs. Approach slowly until your front wheel is at the edge of the curb, and then wheelie yourselff off on your back wheel to get the feel of dropping off. Once you've done that a few times, and you have the feel down, go to your local trail and find something a little bigger. Do the same thing as I described above, and do it many times until you feel comfortable. My first drop on the trail was about a 2 footer. The first time I attempted it, it didnt go too well simply because I was scared. But after a few more attempts I got the feeling down and now its no big deal at all.
The key to progressing is taking your fears and limits then breaking them. Once you beat an obstacle on the trail, you will feel like you can do something bigger.
On yesterdays ride I was down at the trail where I did my first drop and I was messing around on it for a while when two guys riding a bighit and bullit came by. I started talking with them and they asked me if I wanted to follow them to a park in Seattle where they were going to do some drop-offs. I took them up on their offer, and before I knew it I was doing a 3 footer, a 3.5 footer, and then a 4.5-5 footer. Yes its scary, but the only way to conquer your fear is to actually do it. Just work your way up, and with each drop you complete and master, you will have more confidence to pull off or attempt something even bigger.
As long as you can pull a short decent wheelie, you can start doing drop offs. Just remember to land rear wheel first when dropping to flat, and evenly when on a slope. How even depends on how big the slope. And to what degree you want to land on your rear wheel depends on how high, and your form.
Not too long ago I was in the same boat as you, and wasnt sure if I could do the drops. Now I feel like I can do a 6 footer, but Im afraid my enduro will break. BigHit here I come.