A little more info would help.
2 channels, 3? Electric or glow?

You need ailerons to do a "smooth" roll. If you have them, start your roll, feed in a bit of down elevator as the plane starts into inverted flight, then ease it off as it starts to come out of inverted.

If you're going to try and slow the roll down you'll also probably need to coordinate some rudder into it too.

With rudder only, the best you can do is a sloppy barrel roll. Start a little dive, then pull up and throw the rudder over at the same time.

Good luck,
Dennis-