Overclocking raises the amount of heat your processor is going to generate. As long as you monitor and ventilate the heat from the processor so it doesn't get too warm, overclocking is a safe way to squeeze a little more performance out of your processor.
You need a good heatsink, and good CPU temp monitoring software to control this situation.
If the processor gets too hot, of course you can shorten the life, or even kill it instantly. One cause of the processor overheating is you must commonly raise the voltage to get an overclock to run stable. This of course only adds to the heat building problem. Again, very important to monitor how hot your CPU is getting.
Mild overclocking like 1-10% usually can be accomplished with the stock heatsink and not too much worry about anything. If you want more than that, you have to start being more careful about how hot things are going to get.
Yes overclocking can shorten the life of your CPU. If you do it correctly, shoot it might only last 160 years or so rather than 200 years. Point is, yes you are going to shorten the life, but the life span of you CPU will outlive you and your kids, and your grand kids, (or basically the usefulness of the CPU) so everything in perspective.
Now, if you do something wrong, like way over volt, let the thing get way too hot, you can shorten the life of the CPU to like .031 seconds. You have to know what you are doing and watch the results.
Different processors overclock differently. Years ago, processors were pretty much binned at the max they would run anyway, as the competition between manufactures was all about speed and megahertz. Overclocking on them was hit and miss, and most of the time very limited. It would be helpful to know exactly which processor you have so we could tell you what your chances are and how far you could expect to go. Second thing is the motherboard, as years ago most motherboards were not built with any overclocking options in the BIOS. You had to buy an expensive board to get to play with things.
Last but not least, overclocking a 1ghz processor is not going to get you much. Basically you are going to turn a very, very slow processor into a really, really slow processor.