If you're just getting started, a lot of the advice here might seem intimidating. (Or maybe not). To make it easy, just make a list of encounters (you don't have to use them all in the adventure). Some will be quite clearly combat encounters, some will be role-playing encounters, a lot will be a mixture of both.
A hypothetical adventure might have:
-Attacked by thieves (combat)
-Shelter at the inn (role-playing)
-Looking for the hide-out (role-playing)
-Buying supplies (role-playing)
-Invading the hide-out (sneaking, possible combat)
-Encounter the bandit king (probably combat, possibly role-playing)
-Free the prisoners (role-playing, combat with the guards)
Prepare what you think you need to for each "scene" in your story, including what the players might get out of it, a little bit of descriptive text (unless you're great at thinking of that stuff on the fly), and a few minor details to make the setting more interesting (cliches like an orc with a scar on his face are fine, so is a merchant with a stutter). You're going to get used to doing a lot of "winging it" (that's a technical term).
If you let them, the players will extend the most minor events into a lot more playing time. If they're walking down the street, a scene you might have written as filler, let them talk to strangers if they ask to. Always keep your eyes going around the table, and if someone looks really intent on the scene, they probably have something to add. At these points just finish your description without changing the scene (so don't get them to their destination yet if it's the journey that's interesting them), and ask "What do you do [while...]?" or "Any questions?" If the answer's no, move along. But often times the answer will be twenty minutes of amazing play experience that hadn't even occurred to you during the design phase.