There's many guides online. I just started playing a couple of weeks ago too.
On guns and perks and stuff, you should look at what they do. Most of the time you can tell what things do before buying them. There's probably a gun that fits you and your gameplay. I played for the first levels with the first pre-made class. I think all the class that come made are good enough depending on your gameplay.
A good tip is playing a lot, learning the map and stuff. Usually on maps there's places where people hide, and like to fight.
There's some game modes that are better for beginers. For example without killstreaks and that, the less bonus the better since you have less of all.
Also don't buy the higher killstreaks since you at first won't be able to get many kills in a row.
I don't know what other random tips to give. Try to look online and practice a lot.
Edit: Prestige is a way to show how many times you've reached the max level. Once you get lvl 50 you can choose to prestige and you'll trade in your guns, perks, and everything for a prestige 1 rank and you open a new symbol, and emblens. So you start at lvl 1 and have to go to 50 again, then prestige and you'll be lvl 1 but prestige 2, until you reach prestige 15 and be the higher level.