Hello Lybrate user, I will start with yellowish discolouration. You are not brushing properly, brush twice daily after meals. You will have to undergo scaling which is actually cleaning of teeth surfaces with an ultrasonic instrument. This will cost you around 1500 to 2000 rs. Black spot may be either cavity or stain, if cavity then it need to be filled up. Filliing of cavity for each tooth will cost you around 500 to 1000rs. Gaps in between your teeth needs to be treated with braces which will cost you around 30 to 40k. For such condition there is no home remedies, you will have to visit a dentist
and treat it. Hope this helps.