Blackjack is an extremely easy game to play. The rules are really basic and you need to become familiar with basic strategy more than anything else. The main goal is to reach 21 or get as close as possible, the cards drawn have the numerical value they display, apart from picture cards which are all worth 10 and aces, which double up as 1 or 11.
There are other things to consider such as splitting, doubling down and surrendering, however, for a first time it's best to just watch, play simply, and get an idea of how the game flows.