There is a TON of things to learn about the game. its really hard to just post a reply and tell you what you need to do. Much of it is learned along the way, but ill jot down a few key points of the game to get you started. A great place to look is youtube and pro streams on twitch. Watch these guys and listen to them explain what they are doing and thinking during the game, that will help you a lot. Just about everybody does this and gets something positive from it.
1.) buy and place wards non stop all game. know where the enemy is can prevent friendly deaths and reveal opportunities for your team to get kills and objectives
2.)the game is not about kills, its about objectives. kills give you gold to make it easier to get objectives, but the end game is towers, inhibs, nexus. Dragon is a good way to gain an advantage. Stay away from baron at all costs unless your enemy team is 2-3 members down and the death timers are long enough to create a safe window.
3.) killing minions and monsters is the safest way to generate gold and XP. prioritize this over kills while laning
4.) be wary of your positioning in a lane. if you have no vision of the river or jungle dont stand too close. if you have vision of one area, position yourself near that area with the minions in between you and enemy laner.
5.) large minion waves do a ton of damage. avoid trading with the enemy inside their wave when it is large, especiallly early game, you will lose the trade and possibly feed kills. this goes both ways, if your wave is large and they come inside for a trade (as long as the power scales are pretty even) look to go all in and use the minion aggro to your advantage, you will win this trade 9 times out of 10 because of the minion damage.
6.) become one with your mini map!!!! each team has 5 champions...where the hell are they?! you should have an answer to this question at all stages of the game, or at the very least an approximate destination and heading. use the missing ping every single time an enemy walks outside your vision, and ping the direction he was heading.
For champion builds use probuilds.net it records challenger and up players and their builds...you dont have to follow them to the letter, but it will give you a relative path that will be optimal for your champion.
Next best advice..find a gold + player that is willing to take some time and play a few games with you and coach you a bit.
best of luck, never give up, keep at it!