2 biggest tips to start with is
Stay with the group and if someone's lagging behind, turn around to check on them every now and then to see if they're about to get fucked and help them, better split 2/2 than 1/3. Don't go exploring shit even if you see a potion unless you really need it. Also ping stuff so your team can see it easier and listen to character voice calls(assassin nearby and shit like that, means start turning around and see if you can find the fucker, tag him and shoot at him).
If anything big is looking at you, Hold block and spam dodges backwards until they stop looking at you. Technically you can weave in attacks and dodges can be used at the right moment instead or preferably side dodges against certain attacks, but the strat works well enough against about everything other than the flamethrower boss, just dodge to the side on this one but that should be obvious from the long lines of green fire you're standing in if you're not moving to the sides. Biggest mistake I see new players do is trying to attack bosses/chaos warriors when they have aggro, you're just gonna get smashed the fuck away and do shit all for damage. Wait for it to turn to someone else, then smash him in the butt. Turns back to you? Time to block. You can easily take 0 damage just tanking a boss even with a shitty tanking weapon like daggers by just holding your block when you have aggro.
Oh and also I guess don't waste your ammo on trash mobs. Ammo can sometimes be very scarce depending on the map and RNG with ammo boxes, and they're pretty much always the best way to kill specials. If you're out of ammo and you have a globedier or a ratling gun firing at your team, you're gonna be in a lot of trouble.
After that there's a lot of stuff that takes time to get used to, especially the melee combat is pretty interesting, but overall for recruit play just sticking to basic rules makes it fairly easy. Also helps once you get better items and start killing shit faster.