You can't plaster directly over regular drywall. You need water resistant board, usually called blue board, though there are different colors.
And really, I think that a newbie who can't tape and bed a smooth dry wall is gonna have even more problems plastering entire wall smoothly. Mud is lots easier to sand, and you can do some final smoothing with damp sponge. If you are not happy with smoothness of your taped joints, consider a heavy texture to disguise it all.
[CENTER][FONT=Comic Sans MS]Measure twice, cut once.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]Look at the nail, not the hammer. Watch the fence, not the blade.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]