HOW TO SAY "NO" ?

i am a teen and i am so pissed at myself i can't say no to my friends asking me to hangout they are fucking drinkers and smokers i used to do it but i left few months ago when i realized how bad it was ik this situation looks so pathetic i wasn't some serial drinker or smoker but i won't lie i did a lot at one point of my life so just help me out how to say no or what to do i gave jee in January got decent result wanna improve in April i am studying but then randomly one day someone calls i couldn't deny and yeah day ruined flow broken this is soo fucking frustrating what should i do i couldn't figure out parents are a different thing they are patient as of now but i don't wanna disappoint them anymore should i block those friends ? idk idt i can really do something in my life with their company but yeah that childhood friend from kindargarden days shit they say when i try to step up for myself and deny