Learn Russian: Мой брат любит футбол.

Possessive words like "my" must match the gender of the noun they belong to. Since брат (brother) is a masculine noun, we use the masculine form мой. If it were a feminine noun like сестра (sister), you would use моя.

Why is the verb form любит instead of люблю?

The verb must match the subject doing the action. The dictionary form is любить (to love). Since the subject is брат (he), we use the "he/she/it" form, which is любит. If you were saying "I love", you would use я люблю.

Футбол is the object of the sentence. Why didn't its ending change?

You are right that direct objects take the Accusative case, which often changes the noun's ending. However, футбол is a masculine inanimate noun (a thing or concept, not a living being). In Russian, masculine inanimate nouns do not change their endings in the Accusative case—they look exactly like their dictionary form.

I don't see a word for "the" or "a" before футбол. Why is that?

Russian does not use articles like "the" or "a". You just use the noun itself. Context tells you if it means "a football", "the football", or just the sport "football" in general.