Most tomato plants are "indeterminate," meaning they just keep continuing to grow upwards. Even the "determinate" varieties can grow tall enough to require staking. In my earliest gardens I used a big wooden stake, and tied the growing trunk to it.  Later, I started using tomato "cages," round or square wire frames that you place around a seedling to contain it’s core. But these often crowded the growing plants.

My current favorite is the  Space-Saving Plant Stake from Burpee.  These angled metal stakes combine the strength of a tripod with the convenience of a
plant ladder. And they don’t require tying — you just guide the growing branches into the steps of the open framework. And unlike most cages, they’re made of heavy powder-coated steel, durable enough for years and years of use. The 5′ tall version comes in sets of 5 for $58.95 at Burpee.com.