ok so the first thing will need to do, well i always do is just vacum the car, seats everything be thorough.
you will need to by a product called no vac foam carpet sanitiser & deodoriser i buy two one for the carpets/floor mats and one for seats i get myn from coles its like 10 bucks for 2.
this is what the can looks like

i find this fragrence is the best from what ive tried
anyway back to it, just spray it every where all over the floor mats, carpet and seats
when u spray it white foam comes out dont be alarmed it will dissolve, dont worry to much about if u get some on plastic it will just dissolve and it also leaves your car smelling nice for a few weeks, oh yeh after it dissolved your seats will be abit damp not really damp iunno how to describe it just let it sit for abit and let it dry it just depends how heavy you spray.
for me it made my carpet look sh*t loads nicer, the car mats looks nicer in colour, and it brings the colour out in the seats more
let me know what you guys think
cheers
ADZSKI