Hey there fellow aspiring summoner,
As someone who was also a very new player at one point (I've been playing for around a year on and off though frankly I still am very, very bad) I know how harsh a lot of the player base is, not just to new players, but just anyone in general (though new players get picked on worse for the fact that they're new).
They do it for multiple reasons, all of them very wrong:
1.) They've been flamed many times before and some people can't deal with it and turn that frustration on others whenever they can.
2.) They get fed and very ahead of the new players easily simply because they are more experienced and then feel like they were gods all along.
3.) When it's your teammates who do it, it's because they simply assume new players will have the same amount of knowledge as they do and find the fact that they make mistakes people who they regularly play with wouldn't normally do unacceptable and instead of trying to give tips to the player and encourage improvement, they flame them and only make things worse.
As someone who's been through this I know this can turn people away from the game for good. Or at least make you doubt in yourself and put your mood down. What you can do, is simply try to ignore them and focus on your own game. If they keep their bad attitude up, you can hit the tab button, mouse over their champion portrait and hit the "mute" button and report them for "negative attitude" or "Verbal Abuse" at the end of the game If they are a problem, the system will eventually punish them and attempt to correct their behavior.
I should also say, never let all the toxcity get to your head. Not only can it affect your performance but it can also frustrate you and lead you into being toxic yourself. It really does happen, I've let it get in my head before and I have been toxic myself to other toxic players harassing me. I try not to do it and whenever I come close to it I think of the old quote: "Never let an Idiot drag you to his level for he will beat you with experience".
Lastly, I'd like to say that if you really, really like this game then don't give up on it It really can be fun at times and when you can tell you've improved as time passes on it feels pretty nice!
I'm not the best player around, but feel free to add me in game if you want and I'll play games with you if I can and try to help you out with the game as far as what little game knowledge I have!