You should invest in a barbell, a pair of dumbells you can add weight to, and some weights. With those things, you can add some exercises that'll help build serious muscle. At the very least, get a set of dumbells you can add weight to, like MDA said. Plus- If you don't have a bench, don't try to improvise one- I hate to say it, but without one there's not much you can do for your chest, except maybe weighted pushups. Read the max ot link, it'll tell you a lot about working out in general- What to eat, how often to lift, etc-
The Basics by Twin Peak Also, check this link out- More good info.
You can do a lot with just dumbells- If they're heavy enough, you can do a killer and complete arm workout, as well as shoulders, maybe even chest if they're heavy enough- But they do limit you when it comes to other areas. In my opinion, you're going to need access to more equipment if you can get it- If not, hell, an improvised workout still beats the crap out of no workout.
Hope this helped-