Looking up your statistics from the last 10 games:
Not particularly amazing. These past 10 games may not be particularly representative, but it did annoy me a little when you exaggerated your kills and assists.
Your builds don't look terribly bad to me, with the exception of the beserker's greaves on cassiopeia. But you seem to have fixed that.
If you're looking to improve your build look up guides on solomid or mobafire or something.
The champs you have played in the past 10 games are Cassiopeia, Sivir, and Galio. None of these are support champs. And no, your skill in this game is not measured by your kills but by if you win the game or not.
BUT, in your last 10 games, you won 7 games, lost 2 games, and left 1. That's pretty good, regardless of your score. I doubt people would whine on your team if you won, and if they do every time, it's highly unlikely that you will get carried every game (aka if you win a lot, it probably has something to do with you).
All that being said, it might be easier to play someone like Yi, or Ashe, or Ryze. Cassiopeia is supposed to be a hard mage to learn. idk about galio.
Looking at the number of games you have played and given that you are lvl 26, it's clear you have played a lot of coop v ai. This has probably contributed largely to your skill deficit (if there is one) because bots are pretty stupid.