I have antique brown furniture in my bedroom and I want to
re-decorate and make it a beach cottage theme?

I wouldn't touch the furniture, but I'd change everything else.
Pale blue with an offwhite ceiling is a great color combination for a cottage theme. You could also do an off white on the walls.
Because you have dark brown furniture, you need to lighten up in other ways. A light blue duvet set with cream accents or an all white set will make your bed seem lighter.
On your windows, have sheer curtains as opposed to heavy drapes. You could also do wooden shutters, which will really add a cottage feel. If you can change your headboard, you can make one out of old shutters as well. Find a home store that sells shutters or an antique store to create a custom headboard.
If your rug conflicts with your theme or you have hardwood floor, add an area rug with colors that give you a cottage feel. Blue and silver would look great on the floor, and choosing a rug like this
Silver and Blue area rug will warm up your room and detract from the dark wood.
Other small touches: Vases with sand and seashells or solid white picture frames with tiny shells glued to the edges. Cottage rooms are very basic, so don't overdo the accessories