If you have a Favourites folder, tap the app until it wiggles, then drag it into the folder. If the folder is on a different homescreen, put it into the dock (at the bottom), slide to the right homescreen, and drag it into the folder.
If you want to put the app into the dock (the bar at the bottom with apps you use most), just drag the wiggling app icon downwards, and let go of it when it’s at the right place.