Well, both Weaponskills (WS for short) and spells can be used in combos, but not together. If you look at the description of your abilities it will mention what abilities it forms a combo with! For example, Fire -> Fira forms a combo, Thunder -> Thundara another! (read the description of Fira and Thundara, it'll explain what benefits a combo gives to them).

TP is a resource, it is basically "Physical MP". You gain TP by getting hit and using your auto-attack (Also some other abilities, but those say that they do it). TP is utilized by Disciples of War to use their WSes, and it will rather rapidly drain if you're not in combat, but it also caps at 3000, with most abilities costing 1000~1500.

In adition, when a WS is usable in a combo, it will have it's TP cost temporarily reduced to 0. That does not happens with MP cost of spells, however.