First, let me saw welcome to the game and community! glhf
Second, I'll be blunt. Unless you're already heavily into other MOBAs and are just new to LoL, you're not going to catch up to him anytime soon if he's g3 and nearing plat. The vast majority of skill comes from experience. Guides and tutorials and other mechanisms can help, but first and foremost you need muscle memory for mechanics and game intuition for the majority of your decision making. That takes from several hundred to a couple thousand of PvP matches to get to the point where you are in the top 20%-ish of players in gold or higher. Many people believe that when they reach 30, they should just go right into ranked. You can, but honestly be prepared to get reck'd. The real game begins at 30.
Now that the doom and gloom is out of the way...here's what you can do to help grow your skills quickly:
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Don't get discouraged! You're new and you're bad. Just admit it. And you're going to be bad for a long time. I'm bad myself and I play with and against gold/plat/and some diamonds. We're pretty much all bad. Don't rage and don't let ragers get to you. It's a game so just try to enjoy it.
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Try to "main" at a minimum 1 if not 2 champs in every lane and role. Even if it's not your main role, knowing what a tank or anti-carry or support or whatever does will help you in whatever lane or role you do play. It also helps you from getting tilted because your jg isn't helping your lane and you're already behind. There's probably a reason they aren't coming or even can't come because another lane has better gank potential/needs more help/or you're too far behind already/etc. It also helps since many new and low elo players will complain that xyz just sat back int he teamfight....when their role requires waiting until cc is on cd so they can engage and do real damage.
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Practice last hitting! CS'ing is the easiest way to stay relevant in any match even if you go 0/11. Here's a practice sheet you can do in custom matches to improve your last hitting dramatically.
https://www.youtube.com/watch?v=2s97Nm8gEVI
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Spectate others in various elos to see what they are and aren't doing correctly. Watching higher elo games is enjoyable, but it's also hard to take away anything you can do yourself when their decision making is that far above your own.
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Map awareness, map awareness, map awareness. Always check what's going on outside your immediate field of vision. Always = every few seconds do a quick glance.
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Learn the various champs and their power spikes (skills, gear, levels, etc.).
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Always help other lanes whenever you have the means and aren't giving up much yourself. Too many people will go 5/0 in lane then complain that mid/bot/whatever fed. If you're ahead, gank that lane even if it's just to apply pressure and let your teammates back into the game. Or swap lanes - if you're fed, you can at least stand in lane against another fed opponent (usually). There's little point in continuing to get fed if you aren't able to convert that into taking objectives.
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Practice. A lot. It just takes time.
Quick tips..
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STAY ALIVE. Despite common misconceptions, the role of a tank is not to die. It's to disrupt their team to allow for your carries to deal the most DPS they can. And if you're not a tank, that goes double or triple. You can't do any damage or help anybody else if you're dead.
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Going one for one is (almost) never worth it.
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It's worth giving up a little cs to avoid going somewhere dark on the map. Vision first.
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Don't flash when you're about to die. Use it to escape the moment you think you are likely to not be able to get away without it (barring using it to avoid a skillshot).
