how can i fix my bath tub
There are quite a few factors that could be effecting the water flow to your tub. Some of these include pinched plastic type water line, low water pressure, shredded seals inside a single handle mixer valve, excessive mineral deposit in the faucet, debris  deposit inside your water line such as fine sand or other particulate, possibly even a totally clogged water filtration system or a non functioning water softener.

If you are unsure about trying to isolate what, if any of these possible causes, could be the problem with your system then you would be best advised to call in a professional plumber.