How do i restain pine furniture that already has stain and gloss on it(i think)?

Use a furniture stripper on it...I like Circa 1850 Furniture Stripper...it is safe for antiques and works really well without a lot of yucky smell. You will probably need a light sanding...If you want to stain your piece darker, then just add your darker stain to it...if you want to go lighter, then you will have to sand all the colour out (down to the natural wood)...stain the colour you want, and then finish with the product of your choice....is all easy to do, just a bit time consuming, but very enjoyable to see it completed.