There are two main ways of getting build/claim permissions:
1) When you claim an area of land using the golden shovel from /kit claim, you will automatically be given the build and claim permissions for that specific claim.
2) When a player trusts you on their claim using /trust IGN, you will be given the build and claim permissions for that specific claim.
These two methods will allow you to build on a certain claim; however, make sure that the area is claimed or else it may be griefed! Hopefully, that answers your questions. If you are still having problems, feel free to let us know.