J’ai acheté des billets d’pour un match de basket tout à l’heure.

In such sentences, "pour" is often used, so as to indicate the "goal" of the tickets (what you bought them for)

Je ne suis pas un grand fan de basket mais c’est l’équipe de mon université qui va jouer et ma sœur va le regarder, du coup j’ai décidé de reme joindre à elle.

to join someone (for an activity) = se rejoindre à
meet with people at a certain point = rejoindre

Ce sera le premier match que je vais regarder, alors j’ai hâte !

Future for the same reason as "sera"

J’espère que notre équipe va gagnerait, mais c’est peu probable car ils ont/elle a perdu les dernières trois jeuxtrois derniers matches.

j'espère que + indicative mood (and future tense, when the action is supposed to happen in the future)

Mais bref, tant que je peux manger du popcorn demain, je seraiuis content.

Would be more natural to go for the present tense. In sentences such as "tant que", which "link" two parallel actions, the tense of the subordinate clause is also the tense of the main one (theorically), so although "serai" doesn't sound shocking at a OMG-level, it's way better to have a present form