It's tempting to call your broker or go online and just start buying out of fear of missing out. Don't do it, a bull market last for plenty of time to have several legitimate opportunities to get in. 

There is a simple rule to follow and that's a "setup". A setup is a defined place in any market where key metrics and price patterns trigger the initiation of a new position. Depending on the time frame your trading, it could be days, weeks or months before a setup develops. The burden is on the trader to study how they want to approach the market before the 1st trade is ever put on.