I know a password is needed, but how do you attain this password or where do I sign up?? sorry still new to this

Either it is a league server, then you need to be signed up for the league and will get the password emailed or sent by private message.
Or it is a private event, so you need to know who is hosting the event and ask an admin from the responisble team for a password (e.g. Fragmaster is holding an Event each Friaday on the (FM) Events Server. To get the password for the server, you need to be on our teamspeak server, where the password is announced prior to the event.)
The third option is, that the server is passworded because the players on it just don't want other people to enter.

Check the server name and use google to find a website of the team hosting the server. Try to contact them and get a pass.

Most crowded private servers are league related servers.

The best you can do is to find the name of the league using the server(most of the time it'll be the same as server name, unless they are renting the server from another team/organisation, etc), find their website/forum, understand how the sign-up process works, and eventually sign-up if you fancy taking part in.