What is the best way to get rid of weeds like plantain,clovers and dandylions there all over my lawn

Weed and Feed will do it. I've heard that liquid application is better. It may take several treatments to eradicate the invaders of your lawn.

Were do I buy weed and feed

Weed & feed... Roundup might work. If you don't want chemicals and there are patches where the weeds are almost completely crowding out the grass, slit a black garbage bag open and spread it over the ground, secure the edges and let the lack of sun and extreme heat kill the weeds. Then till the soil a bit and replant the grass.