I'd love specific directions! Thank you for your time!

To join the Imperials, go to Solitude enter the big castle. To join the Stormcloaks, go to Winterhold (Windhelm? I get them confused...) and enter the big castle. You should have a misc quest telling you more precisely where to go if you've ever talked to an Imperial soldier or a Stormcloak soldier.

I've been in both of the castles, is there someone specific to talk to and are there pre-requisites?

General Tullius for the Imperials and Ulfrich for the Stormcloaks. There shouldn't be a prerequisite as far as I'm aware.