The most important part is to start off in the correct location. Once you have started the hole it's relatively easy to keep straight if you take it slowly and have a steady hand.
If your drill has a variable speed - either via a selector switch or by varying the pressure on the trigger - use the lowest speed possible to start the hole. A drill that uses trigger pressure is best as you can start with the drill bit on the mark and then gradually increase the speed. This prevents the bit skidding on the surface.
Once you have the first few millimetres drilled bring the drill up to full speed. You will need to push but don't use your full weight on the drill.
You might find it useful to have someone else watch so they can tell you if you aren't holding the drill perpendicular to the wall.