Autocad 2007 questions, there may be more to come?

so i haven;t used a cad product in about 10 years,got auto cad 2007 so i could design a project i am planning. so far i like it, the 3d modeling is great. but all i want to do is draw a piece of channel. can i and if so how do i edit the dimensions , can i Just get my basic shape and then go in a enter my measurements, say i want a piece of channel 1/2 inch across the back with 3/8 inch legs and 12 inches long. there are more dimensions but you get the idea what i want to do.