Just building my rain barrels this spring, so I've read a lot, but don't have a lot of experience, but... assuming your barrel is plastic, you should be able to drill a 3/4" hole and screw in a PVC or brass spigot or hose adapter. Seal with silicone if it leaks.

One thing I can recommend from my rain barrel learning experience. Go with a completely sealed barrel with a fill hose that connects from it to an automatic downspout diverter. Otherwise you have to add mosquito dunks all summer. You still need a screen on the end of the fill hose to keep leaf debris etc from the roof from getting into your barrel unless you put a tight mesh screen over the opening to your downspout in your gutter.