You can purchase siege tanks from the unit shop for 150 minerals. In order to put it in siege mode you must also purchase the siege mode upgrade from the same unit shop in the upgrades category for another 150 minerals.
Siege tanks are very good for holding out the center especially if you have a sizable number of siege tanks and they are combined with a large force of ranged units under a mothership.
You can use siege tanks offensively or defensively- put them behind your bunkers in defence to make a tough defence line, or advance with them slowly in offense to clear out enemy defences. Tanks are particuylarly well suited to this function as they deal bonus damage versus structures. Make sure you advance with them slowly, unsiegeing one or two at a time and moving them forward under the cover of the third tank. Overextending is a costly mistake, but as always in tough situations you can use recall from the support tower to save your tanks.