There are many places you can go to obtain free complete avatars, or components to allow you to customize your appearance. One such place is Caledon Oxbridge University. They have a selection of pre-made avatars for you to choose from, and you can mix and match the outfits and components easily.
http://www.caledonoxbridge.org/
Oxbridge is a great place for newcomers to Second Life as well, as they have a great tutorial area, friendly staff, and hold various classes about topics in Second Life.. all for free.
You can visit Caledon Oxbridge directly here:
http://maps.secondlife.com/secondlife/Caledon%20Oxbridge/92/198/28
Of course, don't forget that you already have a massive amount of free content in your inventory to choose from, and much of it can be adjusted or modified to give you a unique appearance. The tutorials and classes at Oxbridge can help you learn to use this inventory.