To unlock the Welcome to the Club trophy in Call of Duty: Black Ops 4, you simply need to reach level 10 (Sergeant) in Multiplayer. This is pretty easy to do and may well be one of the first trophies you unlock.
Generally speaking you want to select the game modes which have objectives and make sure to go for those objectives in order to earn more XP. Domination and Search & Destroy are good examples.
If you go to the Options menu in the Multiplayer menu, you can tab over to Challenges and see what you can work towards. Challenges span over multiple multiplayer sessions so you can keep making progress towards big XP bumps. Complete a full set for 5000XP.
Once you hit level 10 this trophy will unlock.