Tú and usted can be used as subject pronouns. News, programmes, tutoriaux et forum sur les calculatrices! Ti this is what is called a disjunctional or prepositional pronoun.
News, programmes, tutoriaux et forum sur les calculatrices! That is to say that it is the pronominal form that you will find following a preposition. Ti, tú, and usted are all personal pronouns meaning you.
In your example above, a ti is. News, programmes, tutoriaux et forum sur les calculatrices! Programmes, tutoriaux, forum sur les calculatrices ti Ti can only be used as the object of a prepositional phrase.
News, programmes, tutoriaux et forum sur les calculatrices! News, programmes, tutoriaux et forum sur les calculatrices!