Getting your suspension dialed is one of the most important technical things to know how to do on a mountain bike. It can really change your ride handling once you have your suspension set up right. One of the first things you need to set up is your sag. UBI published a nice little article that details exactly how to do this. Check out the article for the step by step process.
If you are new to working on your own bike or would like to learn some bike maintenance and installation skills, I highly recommend checking out Dave Delgado’s DIY bike repair videos. Dave has a ton of videos on all sorts of repairs, installations, tuning, and maintenance topics for your bike. I’ve used several of these videos myself for a few tricky subjects I wasn’t totally familiar with like help with damping settings and bleeding brakes.