Yogi
The thing to remember about horses is, mentally they are naughty children, and they will start by seeing what they can get away with. If they sense you are unsure, they will play you up, so you have to stamp your authority from the first few minutes on board.
When your horse refuses to walk - a good start - give him a firm kick with both heels and shake your reins, and say "Walk!" in a firm, but not loud voice.
When he starts tossing his head, or looking meaningfully across massive fields, pull his head firmly, but not sharply, around to the direction yo want, and kick him to walk.
After a few minutes of this, he will realise that you are the boss, and behave. Failure to establish superiority can lead to all sorts of anti-socail behaviour - bolting, throwing, rolling, so make sure you get in front first.