Learn Polish: Cały czas myślę o tobie. - I think about you all the time.

The preposition o (when it means "about") requires the locative case. The locative form of the pronoun ty (you) is tobie. Forms like cię and ciebie are accusative and genitive, so they cannot be used with o in this meaning.

Is cały czas in the nominative case here?

It looks like the nominative case, but it is actually the accusative case used to express time duration. In Polish, phrases answering "how long?" often use the accusative without a preposition. Because czas (time) is an inanimate masculine noun, its accusative form looks exactly like the nominative.

Could I change the word order and say Cały czas o tobie myślę?

Yes, Polish word order is flexible. Putting the pronoun phrase o tobie before the verb is very common and sounds completely natural. Leaving it at the very end of the sentence, as it is here, places slightly more emotional emphasis on the fact that it is you being thought about.

Does myślę mean "I think" or "I am thinking"?

It means both! Polish does not have a separate continuous tense like English does. The imperfective verb myśleć covers both the general action ("I think") and the ongoing action happening right now ("I am thinking"). The phrase cały czas makes it clear this is an ongoing habit.