Working with characters
To change the character there is a menu under the stage called new sprite. There you have four options: Choose sprite from library, Paint new sprite, Upload sprite from file and New sprite from camera; but the best option is the first one: Choose sprite from library.
To change the size and the position of the character you need to go to costumes menu next to scripts. There, if you click on the character, you can change its size using the dots on the rectangle around the character. On this menu you have some options. To change the position left-right or up-down you need click on the options Flip left-right and Flip up-down situated on the top right-hand corner.
Changing the stage
To change the stage in scratch you go to looks in the scripts tab. There is a command called change stage. When you select that command you can use all the backdrops that you selected earlier.