Baby first food

Is avocado better to start with baby's 1st food than cereal? I read that rice cereal contains arsenic, hence oat cereal is better to give baby. And do you guys stay with 3 day rule before introducing new food? I read that new studies says not a need for 3 day rule since allergy happens within 2 hours of eating an allergic food? And how do i go about introducing new food daily? By how many teaspoon and how many times a day? Care to share your experiences? Thanks! #FTM #advicepls #firstmom #pleasehelp #firsttimemom #baby