I see where your problem comes in so i'm not trying to diss you but....
1st off DON'T play nunu support its not that good
2ed if you want to play nunu play him jg
3ed if you want to win play freelo champs like: viktor, malz, kat, darius, shyv, fiora, mord ADC, annie, ect <------- these champs are all freelo IF and only IF you can play them. So don't go into a ranked game as a 1st time and feed then get mad at me saying "viktor isn't freelo he is so bad" or "wow thanks for saying malz was freelo RIP my lp" or any thing like that
4th if you want to do support then play good solo que supports like: blitz, braum, thresh, naut they can feed your ADC kills even if your AD isn't the best
5th i didn't check this but last hitting minions is THE MOST IMPORTANT THING in this game. If you can't last hit then you can't get gold, if you can't get gold you get no items, if you don't get items you don't do shit, if you don't do shit then RIP LP. you need to last hit to do anything with top, mid or ADC
6th just have fun in all your games. Don't pick AP zed or anything like that but still have fun when you play ranked.
I was put in bronze 4 from my promos and then went down to 0 LP but i then started to play and play and play with kat and i just kept winning and winning and winning and winning and got up to bronze 1 and had to do 8 promos to get to silver O_o. So just keep trying and have fun :D
PS: Sorry for the wall of text