You need a can of butane lighter fluid with the multiple filling cap. I'm not sure what country you are in but I know they are available at Walgreens.
Be careful, if the gasket is old and leaking it could turn into a fireball so test it outside and don't fill it all the way.
Oh and a little tip, Ronson flints are very hard, Zippo flints are a bit softer which is better for older lighters.
The flint is held in the chamber by a spring/screw so unscrew that from the bottom, slip in a new flint and screw back in place. You will need to 'seat the flint' so try to light and if the wheel freezes just turn it backwards a few times and it should be fine.
The Ronson Lighter company started as 'The Art Metal Works' in 1897 and the Ronson Varaflame was introduced in 1957 and was rather futuristic for it's time.
Ronson invented the first mechanical cigarette lighter and the first lighter with an adjustable flame.