Ha! I love that. I'd start by pruning leaves that are touching the ground. You will see two kinds of growth on a tomato. 1) leaves, 2) stems or suckers. The leaves are pretty obvious, like the leaf on the lower left. It is just a leaf. It has an end point. It is not growing a new stem. The suckers/stems grow out from between where a leaf joins a stem. A sucker is trying to create a new vine, and will set flowers. But in a closed space you don't want too many of them -- maybe 6 or 8 "vines" max in a closed system.
On this plant I'd start by pruning any leaves that touch the ground. Cut them off back at the stem. Then I'd installthe trellis right on top of the current top growth.
You can also clear off any brown or unhealthy leaves. Don't worry about pruning too much, you could take off 1/3 of that plant and still be ok. Post a picture after pruning and we'll see if there are any other clear next steps.
I don't think "topping" this tomato is needed, as it has already developed lots of lead stems, and as an indeterminate variety (that means it will grow forever) you're not going to be able to keep it in the box without aggressive pruning anyway.
You may get a few tomatoes soon! Start checking the older flowers for tiny, bb sized tomatoes in the middle of them.
