How soon?

I'm new to TTC. With our 19 month old I was on BC, but got really sick and had to go on antibiotics. Didn't know they interfered and I was in the midst of a tough semester with my Master's program, so I didn't have any normal symptoms. I was almost in my 2nd trimester before we realized I was pregnant. Now we're actually trying and I don't know how soon to do a pregnancy test after ovulating. Or how often to check. These sticks are expensive!