To write a program, you select a sprite and grag code from the Coding blocks into the Script box and clip them together to do different jobs.
Some blocks just stack on top of each other, but some can be dragged into spaces in each other. So, for example...
In the above, the word "Joe" would be typed in, but the rest would be dragged together.
By and large, if you pick up the top of a set of blocks they'll all move, so to remove a block you may find you need to pull out more than you need and put some back in after you've dragged lower blocks away.
Go on to Some useful types of blocks.