Learn what it means to diversify: invest part of your money in one thing, part in another, and so on, in several different types of companies. This lowers your risk of loss, because some of your investments can lose money, but what you hope is that others do well enough to make up for it.
It is smart to diversify, and an efficient way to do that is to invest in a mutual fund through one of the big name investment companies such as Vanguard or Fidelity. Look online for their Stock market mutual funds. By purchasing shares in one of these mutual funds, you are actually investing all at once in all of the different stocks that particular fund invests in.
Don't invest in anything just because I say so. Read about diversifying and about mutual funds until you understand these terms, then if you agree with me that this is a good idea, go for it.
One more suggestion, if you are in the US and intend this investment to be for retirement, familiarize yourself with the idea of Roth IRA's too.