Hi.
Yes, tanning is incredibly bad for you. Each year, the sun is solely responsible for one million new cases of skin cancer. While most are not life-threatening, many cases are disfiguring. And most people's damaging sun exposure occurs before the age of 20.
For this reason, it's important to wear a sunscreen with a sun protection factor (SPF) of at least 15 for any outdoor activities. This will help protect your skin against both kinds of ultraviolet radiation from the sun (UVAs and UVBs).
Any time your skin changes color due to sun exposure, your skin is damaged in some way. Long-term excessive exposure to the sun, and frequent tans and/or sunburns are very harmful. Sun-damaged skin gets older faster and is a major factor behind the increase in the number of people diagnosed with skin cancer. Tanning can make you look older because it causes the skin to wrinkle and age--kind of like a piece of leather.
However, some people like the way their skin looks when it is darker than their natural tone. There are self-tanning lotions and creams that work by temporarily tinting the color of your skin without you having to expose yourself to the sun. You can apply them yourself or get someone to help you reach areas that might be difficult to reach on your own. They are generally time-sensitive and, like real tans, don't go away for some time. If you decide to try one, it's a good idea to test it out on your skin in a hidden area to make sure you are not allergic to it and that you like the way it works with your skin.
Tanning beds are NOT a safe way to get the look of a tan. They are just as dangerous as the sun itself when it comes to skin damage, and the idea of using one to develop a "base tan" is unhealthy and increases your risks of developing skin cancer.
Ultimately, you might decide that your natural skin color looks just fine and that it's not worth the health risks of tanning and/or the process of applying and maintaining color with a self-tanner. Depending on what you decide to wear on prom night, you may find that your skin looks great the way it is.