Put the 1500 inside a roth IRA. Pick a low cost indext mutual fund and then dollar cost average over the next 15 months. So automatically invest 100 a month. Market will probably go down for a few onth but you are getting two components of diversification. You get product mix with the mutual fund and Time diversification with the dollar cost averaging.
Disclaimer is that more information would be needed in terms of what asset class would be best depending on what you currently have and what your goals and risk tolerance may be.
