What is it constructed from? My shed is brick walls with cement floor so paint was the obvious solution. I used leftover (interior) emulsion in mine, although I've put a tumble drier in there since, and realise kitchen/bathroom moisture-resistant paint would have been better. (Alternatively, perhaps Weathershield exterior paint would work in a similar fashion.)
We converted our garage and kept the back section as a store room so some of the walls are breeze block, parts are in brick and the floor is concrete.
I did exactly what PigletJohn said, we used just plain old emulsion, we had bought a vat of the stuff as the garage was plastered out so we needed a "mist" coat. It was just what was on offer at the time so Crown pure white emulsion.
We rollered it on the walls, I believe it was 2 coats. Then we used Leyland Heavy Duty floor paint from Screwfix and I rollered from the far end and backed myself out of the door. Again 2 coats.
I did hoover the walls before painting and the floor, and the floor again after the first coat of floor paint just to keep it as dust free as possible.
Floor paint is more expensive, and is (usually) thiined with white spirit, not water. Check the tin. Have good ventilation while applying or drying due to the fumes.
Wilkinsons sell the International brand, which is good quality and should last well. It is also sold in the DIY sheds. Light grey or tile red is popular. You have to clean off any grease or oil and let it dry.
To estimate how much you need, measure how much paint you use to paint one wall of your shed. If the floor is about the same size as the wall, you will need a bit more for one coat on the floor (it goes on thicker). You will need at least two coats on the floor.
Your shed will then be much less dusty, and easier to wash down. On rough surfaces like unplastered walls, a large brush will get into the irregularities better than a roller. Do the ceiling too. White emuslion will do here as it will not be subject to wear, but if you bought a big tub of masonry paint for the walls, and have plenty, you can use it up on the ceiling. You will find the shed much brighter and more pleasant.
I'll probably do mine again using PigletJohn's excellent advice I've got masonry paint to redo the outside and adjoining garden wall, so there should be enough to go over the inside too.
And yy to its being much brighter and pleasant inside, it's a job well worth doing. My garden faces north and it's amazed me how much brighter even the garden is for having rendered and painted the outside of the shed - the light reflects so well back into what was a gloomy space.