First of all, nice job for someone who has just started drawing. There's definitely things that need improvement, but for now I'm gonna focus on the anatomy since I figure that's what you're looking for help on. The torso on the bottom sniper is way too short. On a guy, the torso should be 2 1/2 to 3 heads long from ass level to shoulder, even with the wonky tf2 proportions. Image included for basic reference, but I suggest you check out what's available on the resources board under reference pictures to get more of a feel for how the mercs are proportioned out. The hands are also undefined and kinda sausage looking. Big, meaty hands themselves aren't bad but I can't tell where the joints and where they're bending, so it makes it seem like the figures aren't actually gripping anything. You get what I mean?
IDK what program you're using or what you did before doing the line work, but my advice would be to do figure sketches and action lines before laying down details. Basically block out how they'll be posed and such so you can check proportions as you go along. You probably know the whole "bodies are just shapes and lines" building blocks but just make sure you know where and how those shapes are spaced.
All in all, very good for a beginner. Don't think I'm trying to be hard on you or be discouraged with negative critique. Proportions and perspective are one of the hardest things to learn, even once you have more practice under your belt. Hope that helps and keep up the good work!