Typically most folks will keep one main vine and remove all the suckers. That way you can train the plan on to a trellis or other structure to support it. You can also run it up using a string, there's many examples online. Having a plant like this will cause the plant to put a lot of energy into biomass production (leaves and stems) vs flowers (fruit) and your tomato yields will be lower than if you pruned the plant.
If it were me, I'd remove all but maybe 2-3 of the best stems and go from there. In that size container, in an organic system, you'll also run out of nutrients pretty soon and I'd plan on feeding it with liquid fertilizers to get a good harvest. Hope that helps! You have some healthy plants though, that's half the battle.