It's as easy as pressing D provided that you don't rotate your craft's root part (Q, E) either in the VAB or in flight. On the Navball in Surface mode while looking up (light side), due East is the direction along the heavy, densely tickmarked equatorial line in which the numbers go down. If that direction is to the right on the navball, as is the default launch condition if you didn't rotate your root part in the VAB, then pressing D is exactly what you need to do. If it's not, then you need to use whatever combination of WASD puts your heading onto that part of the line.