Firstly, never use the Pompson, it's ♥♥♥♥.
Second, stock is pretty good, don't be afraid to use it.

Now, the actual guide.
for your tele, try placing your entrance near spawn, so your team can see it, and place your exit some near the front lines, but not out in the open so the enemy won't see it.
for your dispenser, keeping it near you for a little bit while setting up your buildings is good for a bit, but don't hog it. Try placing it semi-close to your tele so people will know where it actually is, but don't place it so close that that area becomes crowded.
and finally, your sentry. Most spots in the game are made to be peekable so your sentry isn't op, but there are a couple spots where you can get some kills. Open areas in the middle of choke-points, corners facing doorways, and in windows facing open places are three excellent sentry spots.

Now for some weapon tips. The widowmaker and the Frontier Justice serve the same purpose, decent sidegrades that reward you for good aim and positioning. The wrangler is pretty good, so if you're confident enough in your aim, then try it out. The rescue ranger is alright if you're turtling, but it isn't great if you're trying to play aggressively. The southern hospitality is good at countering spy, and the Jag and Stock are great for many different playstyles. Finally, the gunslinger is the basis of being a super aggressive engineer.

I hoped this helped a budding engi player get some traction when it come to playing the game.