Play games you hate. It'll make you a better player. When I was first starting out in finding competitive pinball, I had serious trouble with slower games, and I'd tend to scissor a lot on >2 flipper designs. I bought a Paragon, which was the worst combination. Now I love that game, I'm a much better player, and it baptized me by fire.

Once I got to playing a bit, I wanted to focus on ball control and improve my flipper skills. I snagged a Pin*Bot (as someone mentioned above) because it has brutal outlanes (set to conservative, no inlane rubber) and you have to play in control and slow the play down (live/drop catch, bounce pass, etc) to have consistent success, but the field is a little open so it gives you time to react and learn. As you get better, keep upping that incline.

Other than that - I'd recommend learning how to work on and repair games. Sometimes it helps knowing how a table mechanically works to translate over to playing it. Helps you remember what machines/manufacturers/eras it's best to employ specific tactics on (ie - tap pass on a Bally pre Williams merger, or holding a flipper in to give an EM kicker a little extra kick)