The roll cast is the easiest cast to learn because it has only one, forward stroke.
I think the roll cast is a great cast to start with because it develops the feeling of a good fly cast. The roll cast is the cast we use when we have obstructions behind us such as trees, banks, etc. It enables us to present the fly even when we have no space behind us or when we have a strong tail wind which would blow a back cast back in towards us or to straighten the line before a pick up.
Always practice the roll cast on water instead of grass, the surface tension of the water helps us to load the rod more efficiently. I use the roll cast like a get out of jail free card from Monopoly, it always helps me out of tricky situations, for example if I see a fish rising or a good lie but there's no room for a back cast I use a roll cast.
The roll cast will be shown for a right hander, left handers should study the concepts and adapt it to their situation. Let's start by getting our selves into a comfortable starting position left foot forward. Hold the rod with the extended finger grip. The most difficult thing for a beginner is how to get the fly line out of the rod tip in the first place, start by striping out about 30 feet of line off your reel now gently stroke the rod from side to side feeding the line out of the rod tip with your left hand, when the line is out of the rod tip grip the line under the middle finger of your rod hand against the cork keeping a firm wrist. Lift the rod and slowly draw the line towards you, bring your hand up high, angle your hand away from your body slightly, keep your elbow in so that the line can move freely behind you and form a loop of line behind you, this loop should resemble the letter D, let the line come to a complete stop. Now aim slightly left of the line in the water, pulling down leading with elbow forcing the rod to load or bend, directing the energy of the cast towards the target finishing with a positive stop about 45 degrees from the horizontal, a loop will form and extend above the water, as the loop unfurls and straightens fade the rod down to the starting position.
I cannot emphasise this enough, don't wait until you are on the water to practice casting, It is impossible to practice casting while fishing, there are just too many other things to concentrate on. Practice on a lawn or a pool free from obstacles and distractions, every hour spent practicing will bring you closer towards fly casting efficiency on the stream.