I'm pretty new to working out, and my workout isn't all that serious. I own a bench press, and dumbells... and basically that is it. What I have been doing is 3 sets of 8 reps of bench press, then 3 sets of 8 reps of 'modified bench press' (you move your hands closer together and it works your triceps more) I do dumbell cells, and a lot of situps (3 sets of at least 50) I have been doing this about every other day. I know this is bad because I am doing no leg stuff.
Using the equipment I have, how can I improve my workout. I want to start working out more frequently, but don't know what excersies to add.

ALso, my school has some workout equipment, and sometimes during gym we will get a little workout in. However, it is not a lot, and they do not have free-weights. Is it ok for me to use something like the tricep machine,and then workout the same muscles a few hours later when I get home?

Thanks for the help.