First off, if the barrel you plan to use to build your project is meant to hold a human or animal or meant to be full of water that might touch a water or food source for a human or animal – use a food safe barrel. You can get the full story about how you can tell what the barrel was used for here. This link has a great overview of where to get barrels and what the barrels probably held before you got them.
The first thing you should probably do is clean your barrel(s) with a mild bleach solution. This will clean and disinfect the barrel. Fill the barrel up about a quarter full. Then add a cup of bleach. (Hold the cup close to the water to pour to minimize splashing.) Use a long handled brush to scrub the barrel. We don’t recommend using a toilet brush unless it’s new. You’re trying to kill the germs – not add new ones.
Vinegar is also pretty effective for cleaning and disinfecting. Borax will remove mold and mildew. Plain old dish detergent will get rid of grease and oil. Then finish up with baking soda to deodorize.
For complete details for cleaning 55 gallon plastic barrels – see this e-How article.