You have several options:
1) Go all in, using a well diversified portfolio, thus less prone to crashing. For example, one of Vanguard's balanced funds. Moderate growth is a good one, or make your own portfolio. This is best if you are uneasy.
2) Go all in 100% stocks, especially if you are going to be making regular contributions in the future. This will feel risky, but historically has been the option that usually ended with the most money.
3) Make small regular purchases, for example $1,000-$2,000 per month to get your feet wet slowly. It feels less risky, but has not historically done as well as 2) in most cases. However, it does as well as or better than 2) about 40% of the time.