As suggested, read the Combat chapter a few times. After that, try to run a few "practice" combats with your friends. For something like this, it's simplest to use characters generated in the Dungeon Master's Guide (DMG), or even mosters from the Monster Manual (MM). Start with low-level monsters if possible. In early Handbook printings, there were some monsters in the back of the book. Anyway, after you play through some simple combats (with minatures, tokens or whatever), reread the combat chapter more, because it's very easy to miss alot of the rules of the various situations. It'll never sink in (well, it wouldn't for me) without practicing. Even ignore some rules, only using the simplest of them first, then add aspects (surprise rounds, move-equivalent actions, ect.) gradually so you're not overwhelmed (again, that was the best way for me to learn, to each their own).
Welcome to the fabulous world of Dungeons & Dragons. The memories last a life time.