Minecraft wiki entry for Beginner%27s_guide, associated text: Movement in four directions, as well as looking upward and downward, jumping, and sneaking. Variations include sprinting and swimming.
As you move around, you occasionally see or produce items floating "loose" in the world. Interacting with those is simple: when you move close enough to them, they fly toward you and you automatically take them into your inventory (unless your inventory is full, see below). At the start of a game, just pick up every loose item you encounter. You may eventually find uses for them, and it takes a little while to fill up your inventory. You can also drop ("throw") items back into the world.
"Mining" or breaking blocks, which is the usual way to collect resources from the landscape. Usually a broken block drops one or more items. Attacking mobile creatures ("mobs") uses the same controls as breaking blocks, and they also drop items when killed. In general, attacking requires brief taps of the relevant control, while breaking blocks requires holding down the same control. Some blocks, such as grass, break instantly.
"Using" items or blocks. This is more complex, since it can apply to blocks in the world or to tools in your hand. The same controls are used for some interactions with creatures (such as shearing sheep or trading with villagers), but this is a matter for later days in the game.
Your character can also work with items in a GUI, especially managing your own inventory, crafting new items, and working with storage items such as chests. This uses the mouse and sometimes the keyboard differently, while you are focused on your inventory and/or a crafting task rather than the world around you.