i usually use the notes app which has a drawing feature for more updated apple devices like iphone 6/7s or ipad pros and ipad airs etc. but uf u dont have those i also use notability(it costs money tho). i also know this app called bamboo paper (free) that i used to use. all of them work very well. if u want a more pencil-realistic sketchy style use notes of bamboo paper, but if u want a more iconic modern and easy style, notability is the best.

this particular drawing was made with the notes app