What are you using? If you are using wordpress, then it can send you the password to the originial user. If not, how can you CREATE a site, and don't know how it works?.
If you have access to a database. Try to open it (adminer.php is a good ideea. Just paste the file on the root and open it. )
Look for a password field. If it's a short password, then it's obvious. If not, then it might be a md5 encoded string. Look for a md5 encoder online. Encode whatever string you want, (for ex "password"), and paste the check code in the database. (remember to save the originial password field, in case you need to restore it.) After you pasted the new key, you should be able to access the site with your new string ( like "password")
