Ripping up the backyard lawn and have a few questions

[socalgardengal]

I will be tearing out the backyard lawn and making a garden area for tomatoes and other veggies. This is my first time ever attempting something this big. Right now it has some grass and weeds. Should I till it or just dig up the first few inches and trash it? I know that I have to add compost but is it ok to mix that in with the dirt that's already there or should I dig deeper and toss it? I guess what I'm asking is what steps would you take to tackle this project? I've read about what to do but I would like some kind of direction. This will be my second season growing and I was using containers before. The area is approx. 20'X10' Feeling a bit overwelmed about starting. Help!!! Thank you