I would like to paint a file cabinet, what is the best way?

I would use a fine steel-wool and scuff the surface of the cabinet real well.... you COULD use sandpaper, but there is more dust. When you have a good scuff all over the cabinet, use a good metal spray , TREM CLAD if you can get it, and spray all over with the drawers removed, with a thin coat. Do it in a ventilated space as the spray can be toxic. Let it dry well, say twelve or more hours, then if the surface is smooth and dry, spray it again with another thin coat. Let it dry so it isn't tacky to touch, then one more spray coat. Let it dry well before you use it. Should do the job for you.

Spray paint it if it is a metal cabinet. Use a good paint like Krylon. Don't buy your paint at the dollar store because it just doesn't go on good. Do a series of light spray coats so the paint does not run. Before you start to paint be sure the surface is clean and dry.

If you have a wooden cabinet I suggest you lightly sand it, apply a primer/sealer and then paint it with a brush.

Get a canvas and some oil paints. Find a picture of a file cabinet to use as a reference if you don't have one. Paint a rough outline of a cabinet. Add highlights and shading.

Use a roller. Just make sure it's clean and dry. See how it covers the color to determine if another coat is needed.