Suzi...
All my trees are trained as single-stem, standard, trees. To turn a multi-stem bush into a single-stem tree will be a work-in-progress that will take place over the next few years.
Select the stem that seems to be the most vigorous, and train that as your main trunk. If it leaves the soil line or is growing on too much of an angle, replant the tree until the main-stem/trunk points straight up. Do what is necessary to get the stem to be straight, including tying it every few inches to a straight bamboo pole, etc. Air layer, or prune off the secondary stem, and get rid of it. Fertilize your tree and let it grow, unchecked, for a few years to thicken the trunk. The more vegetation, the thicker the trunk must grow to support all the new wood, and leaves. After the basic trunk and some branches are established, start pruning away all unnecessary branches that spoil your design concept. Rub out all buds that will sprout from the main trunk. Don't allow suckering. Keep your vision in mind and eliminate everything else that doesn't conform.
This process will take a few years, and training becomes easier as time goes on. Figs are very dynamic growers, and in your climate, you'll probably have a nice 'tree" in hardly any time.
You will get a thicker trunk in a shorter amount of time if you tightly pleach/plait/braid a few stems together and let them fuse, and self-graft. Do this while new wood is still green and easily bent to shape, or, the stems will crack.
If you want a very large tree, I'd start training a good "Black Mission" strain. This variety is known for the very large, ultimate size and height. Stay away from dwarf varieties.
Have fun, and good growing. I'll be starting this process with two new figs that I got last year. Both have double-stems, and will be air-layered to form single-stem trees.
Frank