A small bowl of slightly soapy water and a brush - washing up brush or nail brush. Scrub away - let it dry - if the seats are still grubby then do it again.
Amazon will sell you Autoglym Interior Shampoo and other similar products. Supermarkets too. Vanish - Scotchgard - Simoniz - Bissell.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.