If a tree lacks a good nebari, it still may offer other qualities too good to pass up. The second element to look for is a good trunk. Traditional 'masculine' trees will have thick trunks and mature bark. The more taper you can get the better. A thick trunk with no taper can be dealt with, but one with taper is better. The usual rule for such trees is that the height of the tree will be six times the diameter of the trunk at the base. So already you must form some sort of picture in your head of what the final tree will look like (apologies to Sandy and other Zen followers). At least picture how tall it will be with this size trunk. Will this work? Is there a branch that can be bent upward for an apex? Can the top be broken and a jin apex carved? Will the tree need to be grown out some more before it has sufficient trunk or height? Trees with smaller trunk proportions are perfectly acceptable, they usually have a more feminine appearance, softer and more sinewy. It just depends what you want and what the tree has to offer.
If you are looking for immediate gratification or pretty quick bonsai you must analyze the branching. If this thing is going in a bonsai pot soon, under one year, then some existing branches must be used. The first branch is usually one third the height of the tree. Does the tree have such a branch? It should be one third or less than the diameter of the trunk at the point at which it is attached, or it will be too fat. Skinny branches can be grown out, fat branches are a serious problem and can only be solved over a number of years by growing out the trunk. Choosing or finding a second branch must also include a decision on the front and back of the tree. Usually the first two branches are at the left and right of the tree and slightly toward the front, they make an angle of somewhere around 120 degrees. The third or back branch is usually 120 degrees from the first or second branch but not directly behind the tree. The back branch may also be the second branch and only rarely the first. The other branches can usually be grown out later.
PATIENCE! OK, you have analyzed the tree and it has good possibilities, buy it and take it home. If it is not spring with the sap running, you can go ahead and do some styling, remove unwanted branches, wire and bend others, remove some surface soil to expose the nebari, reduce the top. In general, have a good time. If it is an evergreen do not remove more than one half the foliage while playing with it. Slap your fingers if they itch to remove just one more little branch. If it is deciduous and it is dormant, have at it. If it a species that buds back nicely, and most do, you can work it down to the trunk alone. Check with someone here to make sure the species will support this. Beech, for instance, usually will not. If it is the growing season but not early spring you can easily remove half or more of the foliage with your manipulations. If it is less than six weeks until the end of the season wait until it goes dormant or you risk throwing new growth that will not have time to harden off.
What about the roots? My advise is to leave them alone. This is where beginners get into trouble. They work a plant too much too soon and it cannot support all the changes. Do as much top work as you can and leave it in the existing can or put it in a larger can without disturbing the roots if it needs to grow out some more. Wait for the next opportunity to do root work. You can continue to clip and trim and wire branches and have a good time watching your little tree progress. It will be very easy to care for because it has excess root capacity compared to the canopy. It will not dry out easily and it will be easy to water and fertilize.
If you worked on the top during the winter wait at least until the following fall to do root work. If you worked the top after the spring flush you can work the roots in the fall, but it would be safer to wait until the following late winter or early spring. If you work on the top during the summer wait until the following fall. In other words give the plant an entire growth cycle before root pruning.