If you're not especially thin or especially fat, you should see tightening up in a week or two -- if you do proper exercise at least 3 times a week.
The WAY you exercise determines whether you cause the muscles you have to become fit and "tight", or whether you cause the muscles you have to grow larger.
It has to do with the amount of weight you use vs. the number of repetitions you do.
If you DO decide to do this, be careful you don't hurt your back. And you will feel GREAT in no time! You'll sleep better -- if you don't go to the gym too late at night.
And remember to watch the types of foods you eat -- not necessarily to avoid fat, but to be sure you get good nutrition to feed those new muscles.