I think i might have ADHD?

So i find it really hard to concentrate in cl and when i try to listen to the teacher my mind always wonders off then i realise im not listening then try then same thing will keep happening. Sometimes im listening to the teachers words coming out of her mouth but when i start thinking back to what she just said i can'tt remember. when i sit at home and try to study, i find it soo hard because i always get distracted by every little thing and i have to keep getting up like every 10 mins to get food or go toilet or something. im in year 12 now and it is my last year in school so its an important year, but i just cant seem to understand work in cl and im so behind that whenever i try to catch up and study at home it just never works. I'm really forgetful and i always say stupid things because i never think about what i say so everyone thinks im the biggest bimbo and slow. once i went out with friends to a rave and took these pills which are for people with ADHD and i took like 6 of them and i didn't feel anything at all and my friend said it could be because i have ADHD. i use to party alot and drink, smoke, drugs and i think its kinda fcked my brain up. now i wanna start being serious at school and ive stopped partying and going out but i just cant concentrate and it piissess me off cause i really want to go to uni. apparently smoking weed fcks your brain up too and can give people disorders like ADHD and dyslexia. if i would've known when i was younger how much damage it would do to me i seriously would have never even tried it. do you think i have it and how can the doctors test if i have it or not ?