I was put on a diet when I was pregnant to keep from gaining too much weight because I have a very small frame and the doctor was afraid excess weight would complicate delivery.
All you need to do is eat sensibly, make sure you get all the nutrients that you and your baby need and get some light exercise. Walking is great exercise and not too strenuous. goku's suggestion of spark people is great - I think they have an area for pregnant women. Also I think MH does too.
First I dont think you can control how much you are gonna gain during pregnancy it just happens!! So just take it easy and after the baby is born you can workout and stuff!!
I have not had kids so i dont have an answer but im sure one of the girls here will answer you soon also but i was just going to suggest taking a look at spark people if you are not a member they have teams related to this and theres tons of mothers on there you can speak with !! They also have a related site called baby fit which can also give you some tips!!