Deleting app completely with datas and files on MacBook Air

There's no easy way. Apple and loads of people here will tell you to ignore the traces left over. If you're a bit OCD about it then you need to get a search app (not Spotlight - I use Easyfind) and search for all the files on the HD and then delete them. This is a pain cos there might be dozens of them and they will be scattered over dozens of directories and many of them will be locked so you'll end up in Terminal deleting them one-by-one as a superuser.

There are apps which claim to do this for you, but in my experience they still don't find and delete all the files. You'll see posts on these boards stating that these apps are, effectively, the spawn of Satan which will destroy your Mac and possibly cause the moon to change orbit, thereby destroying the earth too, but they, like the files left over from app deletes, can be safely ignored.