Both methods work — they just optimize for different things. The snowball builds motivation; the avalanche saves the most money.
The snowball method
Pay minimums on everything, then throw every spare dollar at the smallest balance. Quick wins keep you going.
The avalanche method
Same idea, but you attack the highest interest rate first. It costs the least in total interest, though the first win takes longer.
Which to pick
Need momentum? Snowball. Purely optimizing cost? Avalanche. See the budgeting guide to plan either.