Learn Italian: Compro i biglietti online. - I buy the tickets online.

To say you buy them online more and more often, you can add sempre di più to the end: Compro i biglietti online sempre di più. If you mean you buy an increasing quantity of tickets, you put it before the noun: Compro sempre più biglietti online.

Why is it i biglietti instead of just biglietti if I mean I buy tickets online in general?

Italian uses definite articles like i for general categories and habits. While in English you drop the article to say I buy tickets online, Italian usually keeps it. If you wanted to say I am buying some tickets right now, you would use the partitive article instead: Compro dei biglietti online.

Is the English word online treated normally in Italian grammar, and does its ending ever change?

Because online is a foreign loanword, it is invariable, meaning it never changes its ending to match gender or number. It is used exactly like this for singular, plural, masculine, and feminine subjects. You will hear it used constantly in Italy, usually pronounced with Italian vowel sounds.

Can Compro also mean I will buy in this context?

Yes! In Italian, the present tense is very commonly used to talk about the near future or a decided action. If you are discussing plans for a trip with a friend, Compro i biglietti online naturally translates to I will buy the tickets online.