I want to say first that there's nothing wrong with not inking your lineart. Pencil lines have just as much legitimacy as ink lines. They produce a softer look to the work once colored. That being said...
Get yourself a bunch of different pens. Good ones, not paper mate pens you write notes with. Pigma microns, Copic multiliners, a cheap g-pen etc. Try all of them - do some warm up lines - straight lines, wavy lines, sinusoids, circles, varying thickness etc. They all take different levels of practice, especially the g-pen. So take your time figuring out which one makes the kind of lines you like. And really important, make sure to erase over them. Different inks will hold up better to an eraser than others. That's important because you're gonna have to go over any lineart you do with an eraser to clean up pencil lines.
Get yourself some good paper. Printer paper is 20lb text, and isn't really your best option. Get something heavier, like 70lb text, or even better, drawing paper. Deleter manga paper is fantastic. I've never seen paper that handles ink flow better than Deleter.
Finally, take your time - go over your pencil lines slowly and with control. Use your whole arm to make long strokes, and maintain pressure with your wrist.