Need some help getting started in Counterstrike: Global Offensive?

1.How do I acquire guns/cases/crates/etc?

You can either buy them, or you just receive them randomly at the end of a round, after playing a round you may notice a couple of players receive something in the end score screen. Keep playing and you'll get a few things.

The only gun skins you can get at the end of a round are very common ones. To get rarer ones you'll need to open cases (see 3.), win them on sites like csgolounge.com or purchase them from the market.

2.How do I start a match with a gun of my own instead of a preset one?

Go to inventory and for each gun skin you want, click it and select replace ct/t/both. When you start a game then you'll have the appropriate skin on the weapons. You can also access the inventory from in game, escape>inventory, however, you can't access it while alive nor while playing competitively.

3.Once I have a case do I need to purchase a key to open it, if so how?

Yes, you need to purchase keys in order to open crates. You can buy keys from both the community market (Steam) or the in game market. Once you get a case it's very easy to follow the links in order to purchase keys to open it.