 An infinitive has the word to directly before the plain form of the verb, as in to win, to go, and to consider.  To determine what part of speech an infinitive is, look at how the infinitive is used in the sentence.

 She expected to finish the race, but not to win. [To finish and to win are objects of the verb expected.]  To forgive does not always mean to forget. [To forgive is the subject of the sentence; to forget is the predicate noun.]

 The best time to visit Florida is December through April. [To visit modifies time.]  If you want information about computers, that is the magazine to read. [To read modifies magazine.]

 They were eager to try. [To try modifies the adjective eager.]  The caravan stopped at the oasis to rest. [To rest modifies the verb stopped.]

 To plus a noun or a pronoun (to class, to them, to the dance) is a prepositional phrase, not an infinitive.  Be careful not to confuse infinitives with prepositional phrases beginning with to.  INFINITIVE: I want to go.  PREPOSITIONAL PHRASE: I want to go to town.