1. Clean the keg with PBW or oxiclean. If it's the first time you are using the keg, I would strongly recommend disassembling to clean to make sure it is 100% clean then reassemble (don't get the gas in and bev out nipples confused. They look almost the same but are different).
2. Fill partially with sanitizer then give it a shake. Give it a small charge of co2 and bleed some sanitizer out of both pop it valves by taking something pointed and depressing the little dot. You will need to turn the keg upside down to bleed out the gas in line.
3. Empty the sanitizer from the keg.
4. Rack in your beer.
5. Replace the lid on the keg then pressurize to 30psi with co2. Discharge the keg then charge back up to 30 psi a few times to purge out oxygen in the headspace. Charge back up to 30 psi before storage. This will seal the lid nice and tight.
Whether you force carb or naturally carb is up to you. I always force carb because I'm lazy.