to the forum.
If you ask a question, please choose a descriptive topic and give us all information related to your problem.
In this case it would more than helpfull to know the vessel you are flying.
But in any case, the answer to this is in the manual to the ship (or the standard orbiter manual, if you are talking about the DG-S).
Usaly there is a lever in the virtual cockpit and/or the 2D panel. Press F8 to switch between those two and the glass cockpit. Keep in mind that you will need to open the scram doors before you can activate the engines.
Happy Orbiting
edit: I can't believe it took me over 4 minutes to write this...