Can anyone explain when I should use se/te/lhe in Portuguese phrases and when I shouldn't?
Here are some examples of their usage:
João, vou levar-te ao aeroporto. --> Here you can see how -te stands for João, so -te acts as a direct object pronoun. Other examples: João, amo'-te'. Maria, quero ver'-te'. João, quem 'te' foi buscar ao aeroporto? In this case you ask the verb "what" or "who": Who am I going to take to the airport; who do I love; who picked you up; etc.
It can also be indirect complement: Quero dar'-te' uma prenda! What do I want to give? A gift. TO WHOM? To you! So the question to ask here is "to what/to whom". In either case, the pronoun -te will always be used for replacing a second person noun or expression (singular).
The pronoun lhe is for indirect object, for formal 2nd person (the "você", which basically works as 3rd person), and for 3rd person, both singular.
Se is a reflexive pronoun. You use it when the action of the verb falls upon its subject, which is the same as saying object=subject when you use this one. It is the equivalent to the English pronouns myself, yourself, himself, theirselves (or "each other"), etc.