There are many, many ways to express this, and it really doesn't matter which one you choose. But depending on the situations you might hear some of these different phrases more often. In the simplest case, "Let's party!" or "Let's get this party started!" are fairly common to use. If it is a frat party, one bro might say to another, "Let's get smashed/trashed/wasted!" On the other hand, if it is a group of young women, they might just say "Wooooo! Paartaaay!" Hopefully this answers your question somewhat. If it is still unclear let me know.