Nice natal plum, It is bigger than I thought, which is good. You have enough ''tree'' there, that it is worth the effort to tell you what I'd do. Letting it grow will help thicken the trunk.
Since you are new, it will be hard to visualize, but look at the tree, with the thought toward identifying the line of the main trunk and main branches. Don't start pruning now, just look and think about it. Your tree will probably be only 3 to 7 of the branches that are currently there. I would let the lowest branch keep growing long and straight away from the rest, with the thought of it being the ''sacrifice'' branch that is used to thicken the trunk. After it is 5 or more feet long, it will have caused the lower trunk to thicken. You want the trunk to approach 2 or possibly more inches in diameter an inch above the soil, so even though it will look silly for a few years, just let that branch grow. The branches you need for the design you keep pruned short, cause them to branch, develop ''movement'', wire them to shape if ''clip and grow'' isn't enough to get them in position. About half way up you should let a second ''sacrifice branch'' grow, but this one should only be allowed half the length of the first sacrifice, and this will thicken the middle section of the tree. You want taper in your trunk, from thick to thin at the top. Sacrifice branches are one way to do this.
Think about styles, maybe use and oak tree as a pattern, which is a central trunk with branches to the sides, until the upper third, and then the trunk divides to 2 branches, then those divide to 4 and so on to make a broad round crown.
Or and informal broom, like a maple, trunk divides to 2 sub trunks, then the 2 divide to 4, then the 4 divide to 8 and begin to arch outward, the 8 divide to 16, those to 32, etc.
Or do you have a style in mind? It is not a pine tree, it should not be a single trunk with a few branches ending in a pointed top.
At any rate - don't cut now, let it grow out and see if it will bloom. But think about the possibilities, and where you have branches, and where you would need to get new branches to make a style work. The style that needs the least number of new branches on the oldest part of the trunk is the one the tree will most likely cooperate with. Plan on actually styling it next summer, or even in a couple years from now. Thicken that trunk up some first.
Nice tree, keep it growing. Next year you can do some selective pruning. Near Edwardsville, IL is Cass Bonsai, take your tree there when they offer classes. Or join the St. Louis Bonsai Society. Attend a few meetings before joining and see if you like the group. There are a number of very skilled members in the St. Louis group. They have an active number of memebers, even hosted a Shohin Convention. Shohin are bonsai under 8 inches tall, a somewhat specialized sub-group of bonsai. You have some great local resources. You have a nice little nataal plum there.