Start by taking off the water shoots, the ones that go straight up from the other branches. That can be done this summer- called summer pruning. If the trees are very old you will have to prune slowly over a couple or years. General guidelines, if the branches cross, take out the weaker one, but be sure to cut all the way back to the shoulder (where it comes off the trunk or branch). If the cut if very large you may have to paint it over with a sealant to keep bugs out. If the branches rub or are very close together the same applies. It is wise to also take the shape of the tree into account. Plum trees especially seem to resent being prune back too severely. Apples for example are a little more forgiving. It is generally not advised to top a tree to bring it down. It weakens the tree. If you must shorten a limb alway cut just above and outside bud. Also you can start cleaning out the branches that are growing into the tree trunk rather than away from it. Except, for the water shoots it is best to prune in late winter/early spring before the buds swell.