You’ll need to make sure you’re familiar with the rules of playing in a tournament, as they are different than casual chess (ex: chess clock). I recommend watching some tournaments to see how it works (search online, there are many places to watch a video); even read some books. Also solve tactics each day for a week or two before the tournament; it’s better to solve 5 or 10 tactics that are difficult for you rather than a big handful of easy ones that teach you nothing. Be sure to enter the type of event that suits you.