How do I change capabilities

I spent 8 months designing my site before I realised I’d built a sort of blog. Then I thought I might use wordpress and do the whole thing as a proper blog but at this time that seems a little too complicated (I’m a novice at all this). Suffice to say, I’ve now incorporated a wordpress blog into the main site.
My problem is this:- Apart from myself as administrator, I’d like to include 2 groups of users as authors, but when they login I only want them to be able to write and publish a post. I understand I have to do this by altering their capabilities but I don’t know how. Can anybody help?