Team deathwatch is a gamemode unlocked as soon as you complete the tutorial. Team Deathmatch is a mode in which two teams of up to four players each kill each other to reach the goal of 20 kills. If either team fails to reach 20 kills by the end of the time limit, whichever team with the most kills win. This is a very standard game ode, and is a used a lot in other shooters.
In a match, there is always a more attacking team. So, certain strategiess can be made for you, if you are the attacking or defending team.
With attacking, the aim is to rush the enemy. So, they best and most common are close range weapons like shotguns and melee weapons, and also some mid range weapons. Watch out for the other team, especially if they are wielding shotguns or bazookas, as they are great defensive and high damage weapons.
With defending, the aim is to kill the enemy within close range before they get kill streaks. Anti ambush weapons like the shotgun can be a formidable weapon when up close to a enemy, but snipers can also be helpful, as you can pick off enemies before they get close.