You choose a female body when you enroll at Second Life. So you have a female body as soon as you get there. If you have never done that, it can be very exciting just to finally have a female body. (And female clothes.)
But you get a choice of only four female bodies. So you look like a newbie and the same as a lot of other females. So, for a lot of reasons, you don't want to keep that body.
Buying a Body
Believe it or not, you can buy a shape. And skin. And hair. You can even buy the whole package, for about $8 American. Then you will look beautiful. That is what most females do.
You can also get a better shape, skin, and hair for free. They won't look as good, but they will do.
Constructing Your Own Identity
If you have the time, energy, and patience, the best strategy is to build your own shape. Pick someone you want to look like. NOT someone you know. Some actress or porn star or whatever. Then change your appearance to try to look like this person.
You can really change a lot on your body, though not everything. But it is really difficult to look like whatever you want to look like. No problem. At some point, you give up on that goal and fix your avatar so you look relatively pretty.
The point of having a goal is that it leads you to a look that is a little distinctive and will feel like yours. If you just buy a shape, you tend to look a little like everyone else.
Then that's you. In a world where most females are drop-dead beautiful, a few flaws in your looks will work fine. You still want to be attractive, and the flaws make you distinctive and realistic.
Remember, not every woman has size EE breasts that defy gravity. Pick something that looks nice but is a little realistic.
Skins and Hair
You need to buy a skin or at least get a free one. They are too hard to make on your own from the start. But you can probably can modify your skin once you have it.
You probably need to buy hair -- I have never seen good hair for free.
Summary
You are trying to end up with a body that make you feel like it is yours. That's the most important thing.
Your body is attractive, but if most of the females at Second Life are a little more attractive than you, that's okay. It is part of the female experience to be passed by for a more attractive female. It is part of the female experience to get into bed with someone and wonder if he will be disappointed. It is part of the female experience not to feel confident about how you look.
Anyway, if you have a female personality and understand how to please men, you are ahead of everyone.