How are you defining "making your own"?
So you know, you cannot create new avatars from scratch. SL has no skeleton tools. All you can do is edit the existing avatar model via the appearance sliders, and attach other objects to the provided attachment points. That's it.
The best way to learn what all the sliders do is just to play with them. There are well over a hundred of them, and they're quite clearly labeled, so listing them here with explanations would just be a big waste of time. Right-click on your avatar, hit Appearance, and start playing. (Don't forget to save a copy first, if you don't want to lose the shape you've got now.)
As for attachments, the first thing you'll need to do is get very good at building. Head over to the Ivory Tower of Prims, if you haven't been there already, for a very good introduction to how SL's building system works. It's all parametric solids, rather than mesh surface modeling, so if you've got previous 3D experience, it's going to feel clunky to you at first. But it's actually far more powerful than it seems. I just takes some getting used to.
And then there are sculpties, which you can read more about at
http://wiki.secondlife.com/wiki/Sculpted_Prims . Sculpties will also seem a little weird to you if you've got prior 3D experience. They're sort of a stopgap between the existing prim system, and full mesh support which has yet to be implemented. They allow you to create non-primitive shapes from a fixed number of vertices per object (third party modeling software such as Maya, Max, Blender, etc. required). It's a bit like origami, folding and bending an existing surface in 3D space, rather than lofting or extruding a new surface from scratch.
SL's really all about doing more with less. Again, if you've got prior 3D experience, it's going to be a bit of a strange transition for you, for a little while. There's no way around that. But it's well worth the initial frustration. Once it clicks, you can make some remarkable things. Just try not to get discouraged as you discover just how many of your pre-existing habits won't be applicable. Ironically, the more experience you've got, the harder it can be at first. Keep an open mind, and you'll be fine. And if you need help as you go, ask away.
Also, be aware that modeling is only 10% of the process in making a great looking avatar, or a great looking anything. About 75% is texturing. If you're not (yet) a good texture artist, either learn to become one, or team up with somebody who already is. The remaining 15% consists of animation and scripting. Some of the best avatars in SL compensate for the system's limits with incredible animations. The lions and tigers and bears you see running around, for example, are really just that same human skeleton, bent over and twisted to move like an animal. Again, it's all about doing more with less. Get creative.