Firt if that is your first game in unreal, do not do multiplayer. Adding multi is about doubling work needed for code part of game.
Then any shooter can besplit into 3 parts:
- player, weapon selection, shooting etc
- enemy ai,
- enviroment, maps art etc.
So start with first part do well player, shooting and gameplay, use some static dummy meshes as targets, do not bother yeat with maps etc. Make good code for this part.