I need some help with my metal rail.

i never have done that before but i would think sand it with pretty fine sand paper then paint it with some kind of metal paint if im wrong my bad

You just need to get rid of the rust before you paint. There's a couple ways that you can do this. First, you could sand it as you mentioned. I would recommend looking into some heavy duty sandpaper to deal with the rust. Second, depending on how bad the rust is, there are some products that you can buy that remove rust upon application. It might make sense to combine the product with sanding. Or, third, if you can find somebody to do it for you, it would be ideal to have the rail blasted. This will remove any rust and paint that is already on the rail.
Once the rail is ready for paint, there are a couple things you should do. First, make sure you get your hands on products that are made to be outdoors. i.e. outdoor paint intended for steel. With regards to paint, get your hands on a spray paint of some kind, or even an airbrush. Normal brush applied paint will be uneven and you'll catch on it.
Other than that, you should be good. All I do is re-apply a coat of paint every season at the beginning (fall).

I just sanded down the tops of my metal rails and put a coat of paint on them. I used a belt sander with some extra course wood belt on it. As long as the belt is fairly course and made of cloth it should be all right. I took the rust off of 48 feet of rail with 1.5 belts.