Why does my dog keep vomiting? please help im scared...?

My dog is very picky when it comes to food. I am always trying to figure out whatkind of food is best for her.. Now all of a sudden she is vomiting .. i was planning to buy her bill jack from the store but when i came home frm skool my mom had alredy bought dog chow... she will onlyeat dry food if we mix it with some canned food... its really difiicult to feed her because she is even picky with canned food.. ceasar canned food is to expensive to feed to herr everyday... before the dog chow she was eating wellness... but as usuall she only would eat it with ceasar... i also dont want to buy food that is too expensive .. also she is starting to have tarter/paqu buildup tht is staining her teeth yellow... i just need to know a way to solve this becuz feeding her is really difficult she would not even eat for days.. i dont know if she is sick or not but i love her and all but if i have to take her to the vet i have to save up money cuz my parents would nagg about it... i thinks its mainly because of the food she eats ... i dont know what to feed her pls help?