I have plastic water pipes. I have very low water pressure in my kitchen and to my washer . Cold water is fines Whet do I need to do?

Ok, so your hot water pressure is just fine at all other fixtures, right? Do you mean dishwasher or washing machine? Both could be affected by some debris breaking loose from your water heater. Do you have some plumbing skills, enough to remove the kitchen faucet cartridge?