You can buy clothes in user shops with tu, the currency of the site, or in the Fashion galore shop; it's under shops>NPC shops>fashion galore. Once you have clothes, click on the avatar tab>wardrobe, or click the link underneath the picture of your avatar and press "change avatar". 

It can be a bit confusing at first but you'll get the hang of it. If you haven't already you should read the FAQ's and rules of the site, these are located on the left side under the logout button.

Take your time familiarising yourself around the site, some fun things to do include exploring the planets and finding some rare eggs to hatch. On Atquati you can go to the Enchanted springs once a day for some items or tu, the forest on atquati you can collect a flower and explore to find any pets that other users have abandoned. On Relcore there is a trash Isthmus where you can collect one free item a day. Playing various games like precious pearl and egg count to earn tu. In luck games you can also go to the apple tree to get an apple a day for each of your pets.. If you have any questions most of us are more than willing to answer them as best we can. Also feel free to rmail me if you need help <3