early game - focus on farm
midgame - focus on dragon and turrets
lategame - hope you never get to this stage at 850 elo because at least 60% of your team will have no idea how to play the late game (no wards, get caught in own jungle, no baron awareness, doesn't group with team, still derping in lane trying to farm waves of creeps, will base to heal after winning fights)
all game - ward ward ward
if you follow the above formula for early and midgames, you'll carry yourself to 900 elo. not much, i know, but one does not simply gain elo. if you ward correctly, and focus on winning OBJECTIVES (read: global gold), you will win the midgame. And if you do these things very well, you can get yourself to 1100 elo assuming your mechanical skill isn't seriously lacking, and you're itemizing well. (i've seen brutaliser manamune ashe at 6-700 elo)
remember to ward - wards are cheap and they win games. wards lead to dragons, wards allow for safe split pushes, wards will help you catch derps out of positions, wards help you secure objectives, wards win fights before a fight even begins, wards win games.