Firstly, welcome to the Waaagh. If you're interested in building something more competitively focused, take a look at the Ork Tactics thread, which I've put a link to in spoilers.
As others have said, you have to take at least one character to be your word. You have this covered in your big Mek, so you don't need to worry about that. From there you can take up to three of everything that isn't Battleline or an Epic Hero. You can take up to six of Battleline units (Ork Boys and beast Snagga Boys) and a max of one of each epic hero (named characters like Ghaz or Snikrot).
Aiming that you've got a big Mek with a kustom force field and you're taking minimum sized units with everything, then you have exactly 800 points worth of models. This is pretty good as either a jumping off point, or as an army to learn the ropes with.
As far as stompy list go, if you really want to you could take loads of Killa kans, Deff Dreads, Gork/Morkanauts, Stompas, etc and fill whatever points you have left with other units. The world is your oyster. You can still take things like Killa Kanz and Deff Dreads and be reasonably competitive as well as they're pretty solid units.
Most importantly, take what you think is cool and have fun.