It's not about guessing, as much as people tell you it is. Guessing is like 10% of the main goal. 50% is picking up a champion who fits your individual playstyle and practicing with them until you get good (ARAM, Bot Stomps, Dom) and then take them on to Summoners Rift when you feel confident.
So to review:
10% Guessing where to skill shot (if you have one on your champ) and map awareness (watching when other people go missing from the minimap)
50% Picking the Right Champ - if you pick a champ you hate/can't play, you'll fail and you'll be in the 30% bracket for teammates in the next step - if you're not on the right champ, you're...
30% Teammates and the ones that don't help/are annoying.
10% Items and right build. Read up on your champion before hand. Check their masteries, runes, setups. Check where they should be (Top, Middle, Jungle, Carry, Support, Jungle) and then basically the game is about farming creeps and harassing them when they come to farm their own creeps and building as the game progresses. Most people jump right in and expect action nonstop. This is not the case on Summoner's Rift. If you like Instant action, that is why I suggested training on ARAMS/DOMS/PC VS AI(Botstomps) before making the plunge - they are all more 'action' oriented maps that will allow you to learn your character a lot faster than SR. Just keep in mind SR moves with game times being 35ish minutes while arams and doms are usually 10-25 minutes, so the scaling will just happen faster.
Take all these into account and you should be fine.