Scratch

For levels:

Use a variable called Level. When something is completed, such as reaching a goal, change the level by 1. If you want a background to change according to the level, use Forever change to costume # (Level).

There are two types of jumping: one using the change Y by X and then gradually changing the Y negatively. The second way requires the use of variables- such as VelocityY. This method is more realistic and smooth.

Forever
Change Y by VelocityY
Change VelocityY by -1
   If touching ground
      Set VelocityY to 1
         If up arrow pressed?
              Set VelocityY to X (value can be anything- 15 is good)