It depends on how far you can travel. The big ones are pretty far away, i.e. Austin, TX Wichita, NE Chicago, IL. These are considered GTs and have 100+ people ussually.
Then there are smaller ones that are put on at the local shops. I know you have done some of those and it would be a great place to start really learning the tournament layout. Beyond that, there are tournaments that are slightly larger than local ones that aren't on the GT level. You can attend these in places like Joplin, MO.
As far as what armies are competitive at the moment and what you should take the be competitive, I'd take a look at bloodofkittens.com or rankingshq to get a feel for what is winning right now.
Hope this helps.