Customizing your avatar
Each avatar has a shape and a skin, which can be a little confusing at first. The shape is underlying framework and determines how tall, muscular, or curvy an avatar is. Most shapes can be edited, but not all. Anything for sale in Second Life will have a set of permissions, copy, mod, and transfer. Make sure a shape is mod (modifiable) if you want to edit it. The skin determines the features, skin color, and make up, if any. Skins can also have tattoos and other embellishments. Most skins can not be edited, but they will look different if you use a different shape. To edit, right click on your avatar and choose "edit shape". It is always a good idea to make a copy of your shape before editing it, just in case you don't like the result. Generally products will be either "copy, mod, no transfer" or "no copy, no mod, transfer". Transfer means that you can give it to another avatar.
A variety of free human male and female skins and shapes can be found here: http://maps.secondlife.com/secondlife/kumashon%20piyo%20land/124/171/22
There are many blogs and flickr groups on avatar style, which can be found by doing a google search for whatever is of interest. For example, "second life fashion free", "second life animal avatars", "Best male clothing second life".
Where to Find Freebies (clothes, furniture, vehicles, houses, etc.)
Freebies Dungeon: http://slurl.com/secondlife/Deva%20Loka/42/28/54
Free Dove: http://slurl.com/secondlife/Gallii/117/55/32
https://marketplace.secondlife.com/ A place to browse that can sort by price, popularity, number of prims, etc. Sign in with your avatar login, and you can use the Linden dollars from your SL account to pay for items. There are many freebies here too.