Hi, you CAN use whatever grows roots, woody trunks and (eventually) branches. Whether you like the look of the source material and can work with it is up to you, but it's certainly easier to start off with a 3-5 yr old tree (or shrub) from a nursery than it might be to use seeds (take so long to get anything you can work on!). As far as suckers go, they won't necessarily come true (match up) with the tree itself as it looks to you because most are grafted, and suckers come from the root stock, rather than the top part. What does work is being able to plant the tree in the ground for a couple of yrs to fatten the trunk the fastest way possible (and that's it), but of course you need to be using trees 'native' to, or hardy enough to survive (in Vt. where it gets pretty cold). Your burning bush may well work out, but it will obviously be some time before you have something to work on. Pruning (in late winter for most trees) should encourage ramification (more branching from the ends of cut branches) and/or more branches lower down on the trunk (if the species does that... some don't and you need to prune only above at least 1-2 existing branches for development or you'll just end up with a stick). I do think a good idea would be for you to start reading as many bonsai bks as you can, and if at all possible find a local (or at least accessible) club/assoc/soc. so you can attend mtgs and see how things are actually done, and learn as much as possible from members with more experience. Good luck!

Just a couple of points to add to Lucy's.

Many bonsais that look old aren't. There are many techniques used to create the look of age and maturity, these will be described in books. As far as getting going is concerned, find inexpensive materail to work on (garden centres, the wild - with permission) in the early stages of the hobby. There will be failures but all the time you will be learning. It is also a key point to seek out plants that have potential before you start to work on them, and this will come with experience. One final point, when considering branch removal, think it through thoroughly - even sleep on it - it may seem obvious but once removed you can't put it back on...... and I'm sure we've all done it!!