I recently helped my friend with this problem. It sounds like your just using the basic avatar.
Mesh is a solid and good quality clothing. Always buy mesh. Otherwise your just buying textures to slap onto your avatar.
A hud allows you to customize whatever the hud is for. For example, your body or some socks. Change the color, resize, or even turn invisable.
If you wanna make your avatar look amazing, you'll have to first start by making your avatar mesh. Meaning her body will be completely mesh. Some bodies would be Maitreya - Lara (http://maps.secondlife.com/secondlife/Maitreya%20Isle/147/166/23 < Inworld) or Slink (https://marketplace.secondlife.com/p/Slink-Physique-Mesh-Body/6421371 < Marketplace)
Once you buy either of these, look for compatible skins/shapes. Read each one you like to make sure it works. Then, once you buy one, get ready to work it all on.
The skiin itself will try to be on with the mesh, go into the mesh body hud and hide your mesh body.
And after all that, your done. :3 Also, when you buy clothes, make sure they're good with your mesh. Or it won't work. <3