I got my dog from a shelter and they say she is an 8 month old pit but that can't be right I have an 8 month old pit and she is much smaller then him... Also her ribs are showing a little how can I help her gain weight?:)

Dogs vary in size within a breed. It is also quite possible she is a mix or a closely related larger breed.

Questions on dogs' weights are common enough, I prepared a sticky to answer most of them, https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post254171 Read through and post back any points you still need help on.

What labman is telling you about size within the breed is correct. Please read the info he has provided. The only thing I want to add is that if you haven't already done so, it would be wise to schedule a basic health check with your vet. Make sure she is free from worms, and talk to the vet about what you are currently feeding her and what portions she should be receiving for her age and size. A healthy dog should have a little bit of their ribs showing. She just might be the right weight for her size. The vet will tell you that.