If you don't have a stable display, using the knob to adjust the trigger level will probably solve the problem. Be sure the mode is "Auto" by pushing the button on top until Auto is selected and lit. There are lots of details related to the trigger that will be covered later.
At the upper left corner of the screen, you should see T'D, which is short for "triggered" if you have a stable display. If you turn the trigger level knob to set the trigger below the floor of the waveform, this will change to blinking "AUTO" on and off.
Note that as you turn the trigger knob, a brown dashed line shows the trigger level and a tiny brown arrow with "T" inside moves along the right. A couple of seconds after you stop fooling with the knob, this dashed line goes away, but the arrow remains. Also along with the dashed line, you get a message that pops up with something like "Trig Level: 860mV" to tell you exactly what the trigger level is.
Pressing the trigger level knob sets the trigger level to zero.
Also, you may as well note that the trigger controls are in fact labeled "TRIGGER" at the bottom in an almost invisible faint grey. The vertical and horizontal controls also have these almost invisible labels.
If you have tried all of this and still can't get a display, you could resort to pushing AUTO, just in case someone left the scope in some haywire setup, but that button is poison and should be thought of in that way.
Tom's electronics pages / tom@mmto.org