I want to change the cupboards in my kitchen, they are stained and gummy feeling, like the stain is off on some areas. I was told I could sand and then restain the cupboards but want to know how to do this.

If they aren't nice and flat, I wouldn't even try. Raised panels and trim are just too much work.
If they are 'gummy' then sanding will quickly gum the sandpaper, but you can try with a belt sander and start with 60 grit. But you may find that you have to strip whatever is on there.
It is rare for kitchen cabs to be stained only. There is likely polyurethane over or in the stain.

Especially since it's kitchen cupboards,the "gumminess" could be grease buildup. Use a solvent or grease cutter to try to remove it. Then the sanding would go much easier