Hey Miss, and welcome to Res! :3
Obtaining clothes for your avatar is simple. If you click on "Shop list" In the Navigation menu, it'll take you to a number of stores. From here, just click on "Fashion galore", and buy all the clothes you wish. xD
You can also buy from user shops, but Fashion galore is usually pretty good for starters. :3
If you happen to buy something that doesn't show up in your "closet", where you select what your human wears, then it usually means that that item of clothing isn't wearable yet. Eventually, ALL items will become wearable, so don't worry, your not wasting tu. <3
To actually dress your person, you just click on Human Avatar in the Navigation menu. There, you can choose skin colour and eye colour and such, as well as select which clothes you want to wear.
Also, if you have any other problems, take a look at the help link at the top of the Navigation menu. It has a lot of questions and answers that might be useful to you. :3 Or, if anything else, you can always ask other users for help. <3
I hope you enjoy your time here on Res! :3