Does anyone know anything about Magic the Gathering?

I want to get my boyfriend some Magic the Gathering cards for the holidays. He already has some from when he played in high school but mentioned that it is for casual play and the deck he has isn’t suitable for competition.

How can I help him begin to build a competitive deck? Should I buy decks or individual cards? I honestly have no idea where to begin, help!

Also, I noticed on Amazon that sellers are offering card bundles, often in the 1,000s, with “rare” cards included. Would any these be a good choice or would I just be wasting my money?