A good way to go is this...
30 seconds, max out. Repeat
20 seconds, max out. Repeat
15 seconds, max out. Repeat
Take 30 seconds off between each set.
-This is a good general guide for some basic pushups. Once you can donimate that set, and do 80 or so in two minutes, then move on from there. Combine that with sit ups, jogging, some squats... and most importantly, a good diet.
For the sit ups, just max out two minutes. Then repeat that three times. The squats vary a little bit from person to person, but doing 3-4 sets, the last until muscle failure, is a good idea.
For running, check out about.com and find one of their programs to match up with what you can do at the moment. Make sure to take the first week very slow for each of these... ie- just do one set of each on the push ups, one two minute set on the push ups. Do that 4-5 times a week.
Make sure to warm up and stretch before you do this stuff, it'll make you feel better, and is just good for you.
-A lot of people are way past a basic workout like this. But it's a very good starting point for your general out-of-shape guy.(Or girl!) I promise that you'll be feeling a lot better after a 1-2 months of this. It won't bulk you up a whole lot, but you will be tonned up, have more energy, and just feel better.
Edit- Of course it depends on a lot of things. If you are out of shape, doing a bunch of push ups will certainly bulk you up quite a bit. You will never be a hulk or anything like that doing bodyweight ecercises though.