It would help to see a closeup of the trunk base. Bonsai are supposed to look like an old tree, so that means a flared, thick trunk. Unless it has that, the best think to do would be plant it in the ground or a flat grow box and let it develop that base.

Check the "Potting" section of this article:

http://www.bonsai4me.com/Basics/Basics%20LearningToWalk.htm

In large part, your capabilities and understanding of the art and its techniques determines to what heights it's reasonable for you to aspire. I said "in large part" because in bonsai, you CAN set your aspirations higher than your abilities and 'grow into those aspirations' if you can muster an appropriate amount of dedication. The budding bonsai practitioners I help are all encouraged to put some material into the ground to develop into high potential prospects they can lift (from the ground) later when their skills have advanced and they're better able to bring out the materials best potential. For example, the advice immediately above mine seems perfectly reasonable to me, but to someone who isn't familiar with developing a tree or what gives a tree the 'story' it needs to tell, it might be beyond where they are in their understanding. There is a difference between telling you what to do to create a bonsai now and how to develop the tree into a much more evocative tree later. Evoke and evocative are two words than need to be used more often in discussions about bonsai, because a bonsai, to be a good bonsai, needs to evoke in the viewer a story of what has happened to the tree (or not happened) to make it look so representative of something we might see in nature - on the edge of a woodlot, alone in a meadow, overhanging a stream, victimized by savage weather, or on a mountainside battered by avalanche and rock slide.

If you are of a mind to make a tree from what you have there now, you should look toward making the smallest tree you can from the material you have. I can't tell you what to do from a 2D image, but it looks like I would build a tree either from the low long branch on the right that stops about even with the door on the right, or from the much smaller branch that moves left and terminates just above the rim of the pot. 80-90% of the trees mass would disappear with one cut and a jin formed by the stub if it serves as a feature that adds to the composition. If you don't yet have the skillset to accomplish that end, then follow Hemlock's advice, but as you do, make sure you try to get a feel for what sacrifice branches are and use them to the tree's advantage. IOW, you can still be working on building a much smaller tree from a low branch while the rest of the tree keeps growing.

Witness:

A larch ^^^ in a nursery can

The next step ^^^

The branches below the heavier wire will be left to fatten the trunk, they asre sacrifice branches. The entire top of the tree above the wired branches will be removed completely, or a stub will be left and jinned. The branch wired to the right is the first low branch. The branch in line with it moving left will be the new trunk line. The smaller branch with smaller/lighter colored wire that is almost vertical is also part of the new trunk. Just to the left of where the wired, almost vertical second order branch is attached to the first order branch with heavier wire, you'll see a very small second order branch which will also be a part of the composition. The part of the heavy first order branch with heavier wire growing to the left of that small, second order branch is another sacrifice branch that will be cut off flush to the base of that small branch after it has done it's job.

You can see that the tree is still largely intact, but I'm working on a very small tree with a relatively fat trunk. Much of the tree's trunk line is established, 2 main branch's positions are determined, and the 3rd branch can also be seen growing to the rear of the plant. It's also wired in such a way that several options are available when I get a good look at the roots. So, I'm already working to position 3rd and 4th order branches as part of the composition within 15 minutes of actual work on the plant.

BTW - I created the bare spot on the trunk above the wired branches so the branches I removed wouldn't shade the tree I'm actually working on. There is no hurry to remove the top of the tree because it really helps to add caliper to the trunk, and larch wounds heal very quickly.

Helpful?

Al