Just get in; just about never get out.
By that I mean don't try to time the market and don't try to pick stocks/funds, at least not in the beginning.
Here's what I would do (have done):
1. Determine risk tolerance: do you like to win more than you hate to lose? Or vice versa? Also a function of age/time to invest (see #2), but generally speaking, if you like to win, invest a larger % in stocks/stock funds; if not, don't invest much in stocks (keep your $ in the bank which is a bad idea but if you hate to lose, that's your only option).
2. If you're investing (as opposed to trading) for the long term (>5 years) then you should be primarily in stock.
3. Build a core/base for your portfolio: S&P500 index for example. Just buy it; don't worry about if it's up or down, just buy it. As an example: many stocks and the indexes are at all time highs. If that "scares" you then put your money in the bank and wait until they are not at all time highs (again, bad idea but if you can't afford to put your $ to work for you and suffer some potential losses, that's your option).
4. When your core reaches a level you are comfortable with, then start doing the whacky things the posts below will recommend you do. Kidding to a degree but the suggestions to come will include this index, foreign this, total market that, gold, options, etc. You don't need any of those options (ever but especially not in the beginning) to build an "engine" that will power your portfolio.
TL;DR: S&P500 index and forget it.