This is just some tips for beginners
1. Stay away from the enemies turrets, unless you have minions or champion beside you...they will destroy you in a few hits.
2. There are loads of characters to choose from, finding one to suit you is daunting, but don't be afraid to experiment, playing against AI is a great way to experiment with new characters. There are lots of guides to help you understand each character and find one best for you http://euw.leagueoflegends.com/champions
. If you find however the new character is too hard to play just stay beside your own turret and take down enemy minions to boost your gold for better items, so the turret will do damage for you and your not feeding.
3. It is a team game so work with your team and support your team even if it means you will die....I have been in games where our team has killed 20 the opposite team kills 52 but we all stuck together took down the turrets and won.
4. Jungling is a good way to make money, and level up but you need to be playing a jungler or you could die very easy at the early stages of the game, characters like sona bad for junggling, but good support for a jungler.
5. The recomended items are not necessarily the best items for the character you are playing so shop round, keep in mind when you are buying items the type of character you are playing. If you are a character good for damage there is no point buying all your items for defense, buy damage items and maby one or two defense items
6 Dont take on the big purple dragon ( AKA barron) unless the enemy team dead as they could gank you and always make sure you have the whole team with you...wait until you at least level 15 its worth it as it gives you 300 gold and (almost unlimited mana or may as well be) if you kill him
7. 3v3 is smaller map, but if you really want to build skill 5v5 better
8. Some lingo as follows bot is bottom lane....top...is top lane...mid is middle lane...gank is a gang attack , feeding is where you are dying allot , AFK is where a member of your leam or the enemy teams leave early....
9 There is a report button for reporting abuse at the end of each match, dont abuse it