For the brawls, make sure that you communicate with the other members on which fray to take. If you're new, let them guide you.

Contribution is done by donating GP to the building that your guild is currently working on (again, you have to be in communication with your guild to know which one that is). You an get GP on the "Buildings" tab itself, but you need DEC for that.

The Splinterlands-Tutorial/FAQ is quite extensive for most of beginner questions, and will help a lot. Here's the site for Guilds, you can go on from there: https://support.splinterlands.com/hc/en-us/categories/6189943230612-Guilds-Tournaments

As a tip: Don't spend your glint on gold foil cards in the beginning! Use it to get all the common cards in regular foil. When you have all of those, try to get all the rare cards. When you have both common and rare, start leveling them up so you can play in a higher league in your guild.

Also, I recommend getting Discord. Some guilds have active chatrooms there, but even if not, you can do groupchats, and follow the splinterlands discord-channel. There, too, you can find a lot of information.

Enjoy playing!