The Knight class has some nice features (Bulwark of Defense, Vigilant Defender, Bond of Loyalty), but some others are totally meaningless (Shield Block, Mounted Combat, Daunting Challenge). All in all, the class is a bit lackluster and suffers from MAD if you want to make full use of the Knight's Challenge abilities.
Here's a Knight handbook for your perusal.
I'd recommend getting no more than 5 Knight levels, and spend the rest of your levels in classes that can actually fight, and make use of the more tanky Knight features. Crusader is probably the best fit, and there's no reason at all not to dip some Warblade, as well.
5 levels of Knight nets you Bulwark of Defense (the best Knight ability) and Vigilant Defender which has good synergy with this, two bonus feats (one of them worthless except as a PrC prerequisite, the other OK at least), medium Armor Mastery (for your Mithral Full Plate later on), a bit of a bonus from Fighting Challenge, and Test of Mettle (which can sometimes be nice).
I heavily recommend getting a reach weapon. Guisarme plus Armor Spikes is actually better than a Spiked Chain IMO, since it saves you a precious feat. Combat Reflexes is a must have, and needs at least 14 Dex to be useful. Improved Trip would be nice, but Int is a dump stat, so you probably won't get it unless you roll for ability scores and get some very nice ones indeed. So for battlefield control/lockdown, I'd say go for Stand Still.
Extra Granted Maneuver and Stone Power are nice once you get Crusader levels. From level 12 or 15, you might want to retrain Stone Power into Power Attack, though, since the temporary HP from Stone Power are dwindling in effectiveness by this point.
Law Devotion is probably the best feat to give you some extra oomph, and can be gained alongside Knowledge Devotion from a 1-level Cloistered Cleric dip (which also provides the skillpoints and class skills to buy some knowledge skills). This is strictly for better damage output, though, and not needed to fuel your role as a tank.
Another interesting choice in a tank build is dipping some Binder. Dahlver-Nar (Shield Self, some natural armor) and Savnok (great armor for free, some little DR) make great vestige binds for a tank.
Ability scores I'd arrange somewhat like this: Dex/Con (both at least 14)>Str>Cha>Int>Wis. Having at least Int 10 would be nice so you have at least two (three for a Human) skills to go with. Wis can be safely dumped since Knights have good Will saves, and Crusaders get Cha to Will saves on top of that. Str isn't really needed higher than Dex and Con. If you can, make all of these 14 at least. In 32 point buy, I'd go Str 14, Dex 16, Con 16, Cha 12, Int 10, Wis 8.