Hi Natasia and welcome.
Do you remember the email you signed up with, or can you not remember the username?
If you can remember the email, but not the password, then follow these steps (I just tried this exact method and it worked):
1. Go to thesims3.com and click "Log in".
2. Click "Forgot my password" or something similar.
3. Enter your email address.
4. Go to your email, wherever it's located. For instance, my email address is a Windows Live address, so I go to windowslive.com to sign in.
5. In your inbox should be a password reset email from EA. Open it and follow the link inside.
6. Enter a new password. You will need to enter it twice, once to tell thesims3.com what it is, twice to confirm it.
7. Press submit.
8. Go back to your email and there should be another email telling you it was confirmed.
Hope this helps!
EDIT: And this is pretty much exactly what SassySimmer said.