I just want to expand on what cup0gloom said. That's a great tool to have, especially when you want to compose something because a lot of times, you get some melody or something in your head, but you can't figure it out. A great thing to be able to do is imagine a line or melody or chord progression and be able to play what you have in your head.
Also, a general rule (well, not rule, but classical suggestion) for chord progressions is this: "Down a fifth, Down a third, Up a step." What that means is that from whatever chord you're sitting on, you can go down a fifth from that chord, down a third from that chord, or up a step from that chord. Things like iv to ii, IV to ii, V to iv and the like will never sound bad. Of course, you don't have to stay within these parameters, but you can never go wrong! Oh, and you can also go from I to anything and from anything to I. See if that maybe helps you out a little bit.