People tend to think Junk is ezmode, which he somewhat is, but only because other players are stupid enough to run into his line of fire. The whole point of Junk is area of denial. Spam those choke points, spam one of the ways onto a point etc.
Tip #1: Go into training mode, and try hitting the moving robots only by hitting other things around you first.
Tip #2: Learn to use your mines for traversal, and landing on top of people, do damage, and mine out again. You jump with a mine, then in the air throw your trap on top of or in the middle of the enemy team, shoot a few grenades and mine out again. Also use training to learn to aim your mines.
Tip #3: Predict. Kinda hard to learn per say, but you can kinda learn this just by observing your own team when attacking/defending, and watching the enemy team. You’ll learn where and when most characters decide to go someplace. Example, Hanzo and Genji almost always goes through the little perch on the side of Hanamuras first choke. Trap, mine, boom, dead. The most satisfying kills with Junk are the ones that the enemy usually doesn’t expect, and which can be done without even making them aware that you’re there.
Tip #4 when using your ult, get the first kill you can if you haven’t got a plan for it. Most of the time I use it to spread the enemy so my team can thin the herd. Sometimes I use it because I saw several people clustered together, but when the wheel gets there, they’re gone, just search out the first one you see and detonate. Better to get one kill, than lose the wheel. Also, stal the wheel before going in for the kill, make the enemy panic before it comes.
Tip #5, when pushing, and there are shields, spam shields, and actually run onto the face of the shielder (Rein, Orisa), you do a lot of damage that way, and if it gets hairy, just mine out. Always mine out.
Good luck though.
Junk is a really fun character to play when you know how to play him, and just ignore everyone saying he’s “easy” to play. He’s not the hardest, but not the easiest either, specially if you want to use him to his full potential.