For the SL beginners:
When you first enter Second Life, your mesh avatar has a regular skin. This is called system or classic skin.
You can buy new skins made for system or classic bodies†. You unpack and wear them.
These skins are NOT visible. They are under the alpha that hides your body and head so you can wear the mesh avatar.
If you take off both the mesh avatar and the alpha, you can wear the system skins.
† These terms are used interchangeably.
For the MESH beginners:
If you have been in Second Life a bit longer, you might want to switch to a brand mesh body and/or head.
There are many brands of male and female mesh bodies and heads. So where to begin?
Shopping for MESH
We suggest you get as many DEMOS of mesh heads and bodies you can find. Try them for a few days. It will soon come to you which head or body you like best view-wise, and which HUD is easy to use for you. Try on clothes, shoes, makeup, other accessories.
Keep in mind not all designers make products for all mesh bodies!!
Once you have chosen your mesh head/body, find an applier for the brand you have OR an Omega applier if your head/body is Omega supported at the 7 Deadly s[K]ins store. GET THE DEMO of the skin applier and see how it looks on your mesh head/body.
Sometimes you may see a line around your neck this can be fixed by using the neck fix option on your mesh HUD. Make sure your body fat level is around the same as the rest of your body to ensure a seamless finish.
More info here: http://meshbodyaddicts.com/mesh-body-reviews/
List of Omega-compatible mesh bodies here: http://slpoweredbyomega.com/supported-meshes/