Throw your leg over the top tube and stand on the ground with both feet. Note that in order to do this you will likely have to tilt the whole bike towards you as you step over. You should end up standing forward of the seat, not trying to sit on it. Take whichever foot you are going to start out pedaling with (in my case, usually the left one), hook it under that pedal, and lift the pedal up by back-pedaling until the pedal is up at about the 11:00 o'clock position - just forward of top-dead-center. While still standing across the center tube - not sitting on the seat - put your foot on that pedal (clip-in if your using that type of pedal) and give a push, which starts you moving forward. As the bike begins to move, and your pedal goes to the botton of the stroke, lift your butt onto the seat and put your other foot on the pedal and just start pedaling.

To come to a stop or to dismount, decide first which foot you are going to put down on the ground. This is important if you are stopping where, for some reason, the ground under one side of the bike is lower than the other, such as at the edge of the pavement where there may be a drop-off onto the shoulder. You want to put your foot down on the high-ground side (ask me how I know this!) Anyway, just before the bike comes to a full stop, take your "ground" foot off of the pedal, push the "other foot" down on it's pedal, and use the "other foot" to lift your butt off of the seat and slightly forward (stand up, sort of) so that when the bike comes to a stop you have one foot on the ground and the other foot on a pedal at the bottom of its stroke.