At the beginning of combat on your turn, you may sacrifice another creature. If you do, Dire Fleet Warmonger gets +2/+2 and gains trample until end of turn. (It can deal excess combat damage to the player or planeswalker it's attacking.)
While resolving Dire Fleet Warmonger's ability, you can't sacrifice multiple creatures to give it more than +2/+2 and trample.
You choose whether to sacrifice a creature and which to sacrifice as Dire Fleet Warmonger's ability resolves. No player can take action between the time you make that choice and the time it gets +2/+2 and has trample, if appropriate.