If you have no clue what you're doing I would honestly suggest going to a game store and asking if they have any free intro decks lying around.
They're 30 card decks that Wizards gives to stores to give out for free that contain only the most basic mechanics.
If you're up for something a bit more involved I would suggest either buying a pair of Commander pre-con decks or buying some boosters to build sealed decks with, possibly see if the store has any pre-release kits from the last pre-release for sale.
Elijah Mitchell
You can make a cube for well under $100 if you stick to commons, uncommons, and cards printed in Conspiracy. You can't really do much else with $100. A few Pauper decks no one plays it or some standard shitbrew, but that's about it.
David Peterson
I wish this were true because I feel like if WOTC actually felt some competition pressure from other TCGs maybe they wouldn't be such an absolute shit-show.
I want Legacy to survive but it can't with how WOTC is treating it.