If you plan on staying in Indonesia for less than 30 days, you don't need to worry about a visa - you'll be stamped into the country on arrival.
But trust us when we say you'll end up wanting to stay in this country longer! The best thing to do if you think you might want to travel (or chill) in Indonesia for longer than a month is to either get a visa on arrival, or apply online in advance for one. This type of visa can be extended for another 30 days at an immigration office or with the help of a travel agent. *Visa and entry requirements are subject to change at any time. Please always check the current status with your applicable embassy prior to travel.