A young horse who is started properly will have no problems moving forward.
When I start one in the round pen, they first know that cluck/kiss means trot/canter. They have a general idea then when I get on about forward. Leg pressure for forward comes secondary.
It's about making it easy for them. Most colts want to lock up and stop when you get on, so you have to free them up. I usually carry an over under so that I can encourage. If they are used to the lunge whip, they will know what this means. It's cluck, and let them go forward. I want them walking freely. If they do not I will encourage them with the cluck followed by a light swat with the whip. I don't knock them around or anything, usually you don't have to, but the light swat is enough to let them know there might be consequences.
I trot and lope mine out a lot. I don't just lope two circles and stop, I lope five or six or seven and then break to the trot. If you stop them too soon, or stop them every time, they will lock up and hunt the stop a little too much. The pressure comes off when they are moving forward.
Forward has to come before anything else, otherwise you can't really accomplish anything. Once they are free moving around the round pen, I will start to figure eight them in the round pen or make smaller circles. After that, I will ride them in the big arena and it's the same deal - Around the rail first, figure eights second, then more advanced steering. By easing them into it they learn they can move forward and steer at the same time. Usually by that point, when you are teaching lateral movements, they understand what forward means.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.