In starting your journey down the bonsai highway there are a couple of things you should know. I have posted this before but it bears restating here, the most important single trait in a bonsai is the size of the trunk and its shape. These are the two most difficult things to design and encourage without a lot of time involved in the process.
You have heard the term "movement" used in conjunction with "trunk" before in this thread. Movement is the subtle or not so subtle bends and twists a mature tree tends to exhibit in relation to its trunk. Generally speaking, movement of this nature begins when a tree is very young and flexible, and in bonsai it is generally done with very young trees. It is difficult to impart movement to a tree's trunk easily and is usually beyond the imagination or skill of a beginner, requiring a bit of experience and proper equipment; no insult intended just telling it the way it is. But there is hope, so keep reading.
Another problem is thickness of trunk. This too can be artificially induced by growing a tree in the ground, or larger container but it does take time, effort and the knowledge of what you are doing and why you are doing it. The main problem is getting both a thick trunk, and movement.
To understand what I am talking about here you should spend some time studying photos of mature or at least nearly complete bonsai and pay particular attention to the size and shape of the trunk. Many of us look and drool over some of these trees but we only admire, we do not analyze. Analyze what you see, and understand that these trees do not become what they are by themselves, 99% of them have been totally manipulated by some dedicated bonsai artist somewhere and sometime.
So----unless you want to spend the next twenty-five years trying to grow a skinny trunk into a masterpiece trunk just to find out you have wasted twenty-five years trying to grow a skinny trunk into a masterpiece trunk---- you need to find material with larger trunks, and preferably with some movement.
Second secret of bonsai. Unless you happen to be able to afford Yamadori bonsai you are kind of stuck with some sort of comercial grower of material useful for bonsai, or just an ordinary nursery. Most of all look for trunks keeping in mind the trunks you have seen in the bonsai pictures you hopefully have studied. Dirty little secret #3 Most bonsai are not grown up into masterpiece bonsai, they are hacked down into masterpiece bonsai. You start with much larger material, you identify the best traits of trunk and movement, or the potential the material has to offer then you cut the tree down to that proportion.
Trunk size is only a matter of ratios. If I have a trunk that is two inches across and the tree is five foot tall you would laugh at the smallness of the trunk. However if I have a trunk that is two inches across and a tree that is eight inches tall then the trunk starts to take on the proportions of a mature tree with in that parameter and its ratio to the top of the tree.
So in a nutshell you have been given the fruits of years of bonsai experience . If you learn this you will save yourself many years of fruitless effort, trial and error. Not that you wont make mistakes, at least your mistakes are going in the right direction.