Yeah, first step is to pound a strong ground stake in the ground at an angle opposite the way it's leaning. I usually just use 2-3 ft rebar pounded in by 12-18 inches (but make sure it's not sticking up where people can trip -- old tennis ball is a good cover for the end that's sticking up -- bright color and padding). Another way, and what JONA is saying, I think, is to use something like taller U or T fence posts as stakes to support the stake lines from the upper parts of the tree.
Then what I use is a piece of old garden hose threaded with a strong rope -- I actually use haystring doubled or tripled but clothesline works too. Use the garden hose as padding against the trunk of the tree, tie once (good square knot) to curve the hose into a teardrop shape around the trunk (not tight -- leave plenty of room to grow.
Now have a helper push the tree upright
as far as it would go without breaking anything including the roots and secure the rope tight on the ground stake. You *may* need to do this from two angles rather than just one directly opposite the lean depending on the size of the tree and softness of the ground. If that's the case, then the first directly opposite was just to get it in position so you can secure the two other stakes.
Over the course of the next couple of years, push the tree upright some more and tighten the stake line several times until the tree is as straight as you want it to be.
When you straighten the tree, you'll notice that some branches/shoots that used to grow in desirable directions aren't any more. I would take out the ones that would now be considered watersprouts -- growing straight up -- because they will take over due to tendencies to apically dominance during this growing season. Train the others like JONA said.
P.S. JONA - please don't hesitate to speak up if I have this wrong or if there's a BETTER way.
