"First blood!" as detailed in the description is the very first mission you play after the tutorial. If playing on the lowest difficulty you will only encounter eleven enemies all on foot.
Best advice for this mission is to use cover and grenades often when possible. Grenades are your absolute best friend against these enemies and if you are sure the grenade will hit, it'll kill them in one shot.
Don't rush down the middle--the Germans have the entire thing ambushed. Flank around the outsides leaving the bulk of your troops in the middle to hold down any German advancement.
Take your time. This first mission will help you get acclimated to the game. Patience is key, use up all of your troops movements and shoot when possible.