To begin a project, select "Create" from the menu bar. This will open the Scratch workspace. You'll see a box on the right called the preview window (it usually starts off with an orange cat on a white background) and a box below it containing any sprites and backgrounds used. On the left, you will see a list of programming blocks, and the middle of the screen is a coding area where you will create your program by combining blocks into routines.
To change or add a sprite, click the blue cat symbol in the bottom right corner of the sprite window, then select the "Choose a Sprite" menu option. You may also remove a sprite by clicking the trash symbol in the upper right corner of the sprite image.
Once a sprite character is chosen, orders can be dragged from the block list area into the middle code area, allowing the sprite to carry out the commands.