Answer:
Use a p2p programme to search and download a zip file with loads of accessories, cloths an skins etc. Unzip and place in your support directory within the sims directory.
The downloaded items will be available in the game with an asterix visible.
Some items need an expansion pack to work and some need installing by double clicking
Hope that helps, there are many web sites with this info and also downloads, just search for sims2 with Google
If its the sims 2 pets then all you have to do is go into the neighborhood screen. Once your there type in boolprop testingcheatsenabled true. Then go into the create a sim. Once your there type in a capital N and then randomize the sim and then you can change it. The N doesn't have to stay there (you can change it) but if you type a capital M then it takes it away.
If you type in the capital N like it says in the previous answer, it will give you custom skins like vampire, zombie and mannequin and it will also give some cool clothing and masks! Its really neat.