Welcome to FT! You've taken the first important step by joining us.

Actually, there's no dificulty in joining FF programs because they're free - so you can join them all.

Realistically though, if you want to maximize the benefits you need to find one and stick with it. Most people choose 1 (maybe 2) based on where they live and where they want to go. For example, I live in the DC area which is a UA hub, so I do most of my travel on UA. If you give us some info I'm sure suggestions will be offered.

After picking a carrier start with the FAQs on the appropriate FT page and study up. You'll find FTers very helpful. There are 2 things to avoid though:
1. Don't cross post (post the same question in 2 different threads).
2. Do your homework, i.e., search for the info you're after in previous posts before starting a new thread.

Again, welcome!