I want to make a registration form and connect it to my website database. Then use this information to log in. Are there any plugins or any other method?
No. What you're asking is some very custom code and rather difficult, it would vary by system. If the other system you're trying to connect to is open source and widely used, then chances are *yes*, there is a bridge out there that you can download. However, since you havent specific what the other system is we can't really help. If it's just a generic database from something in-house, then no.