We took a tour (by bus) from our hotel. I'm sure you can get there more cheaply by public bus, somehow, but I doubt if it's worth the hassle and wasted time. (Especially if you aren't fluent in Chinese.)
Oh, but our tour didn't include the Ming Tombs (as some Great Wall tours do). I think it's because they're too far away from Simitai.
we've actually been to the wall today.
we avoided badaling and opted for the mutianyu section. this section is also quite touristy, with a cable car to the top and a tabogan back down. but that doesn't detract from how spectacular it is. we actually only arranged the tour yesterday through an agent at the north end of the forbidden city and paid 100 yuan each to get there and 40 yuan admission. we could probably have got there cheaper by bus, but this took the hassle out of it for not much extra. if you can visit one of the other sections of the wall then i'd go for that, but mutianyu is a viable fall back option.