Good advice Kevin.
Make sure that they are supple to the bit and willing to bend their neck to each side with just the lightest pressure before you EVER get on. Make sure that you have established a cue for forward motion before you get on too, whether it be a word, smooching, or clucking. If one of them starts to buck, don't pull with both reins, that helps them buck harder. Tighten one rein and urge them into fast small circles. Get their feet moving in the direction you want and they will usually forget all about bucking. When a young horse is a little tired is when they learn best, don't be afraid to get them sweaty or ride them for more than 30 minutes. Just watch for their signs that they are ready to stop and don't push them beyond what they are ready for.
And if you think even for one instant that you are in over your head, get some help from someone with more experience on young horses. They are challenging and no matter how many of them you've dealt with, they will still suprise you.
Good luck and let us know how it goes.