-IR verbs belong to what is called a "regular conjugation," by which is meant simply a large number of verbs that have the same endings (or are "conjugated" identically). Recall that you have already learned two regular conjugations, -ER verbs and -RE verbs. In the present tense of the indicative, these forms are, for instance:

Parler
Je parle     Nous parlons
Tu parles   Vous parlez
Elle parle   Ils parlent

Attendre
J'attends     Nous attendons
Tu attends   Vous attendez
Il attend      Elles attendent


-IR verbs too have regular, predictable forms. So, for example, the present tense indicative forms of the verb finir would be:

Je finis    Nous finissons
Tu finis    Vous finissez
Elle finit    Ils finissent

The present tense indicative forms for maigrir would be:

Je maigris    Nous maigrissions
Tu maigris    Vous maigrissez
Il maigrit      Elles maigrissent


To form the past participle of the passée composé for -IR verbs,simply drop the -R from the infinitive. So:

J'ai fini    Nous avons fini
etc.

J'ai maigri    Nous avons maigri
etc.

Notice that for the third question ("Quels plats est-ce que tu as choisis au restaurant universitaire dernièrement/récemment?"), the past participle ends in an -S. According to the conjugation rules, its should be simply "choisi." Here it takes an -S only because it agrees in gender and number with the preceding direct object "plats" that happens to be masculine plural. For the answer, since the direct object would follow--not precede--the past participle, the spelling would be: "J'ai choisi...."