The Scavenger Achievement is obtained by mining 20 different kinds of items. It is typically the first achievement a player gets, as its requirements are very simple.
Master Scavenger Achievement
The master version of the achievement requires you to mine 100 different kinds of items.
As a new player, the easiest way to find 20 different items to mine is to find a decayed structure on the surface of your first world. A ruined house will have several kinds of decayed building blocks, fences, and other such items. You can also just dig straight down and find several kinds of ores or underground structures to mine.
Each time you mine an item you've never mined before, you'll see a yellow XP notification text on the screen.