You game needs stuff for your hero to avoid (spikes, fire, lava, bad guys, etc.) And things to collect (coins, keys, treasure, etc.) The code is going to be custom to your game, but should look something like this:
This code will make the sprite hide until level 3.
This code makes the sprite show up on level 2 and disappear when it is touched.
For each item you also need to add a block to the touching stuff custom block.