Experience is the main thing you need, play and get used to how the game works. Think of LoL as like a sport, if you're just learning to how dribble and shoot a basketball, you not only will be bad at them since you just started, but you'll be too busy concentrating on that to be able to handle complex plays and tactics.
A few things to focus on as a new player
1. Making gold (last hitting), while there are exceptions, most games are determined by which team gets the most gold (and therefore items), the main concept to getting gold, is learn to get the killing blow on enemy creeps (the little AI controlled dudes), be wary of constantly attacking them, it will often push you farther out (meaning you're easier to gank) and also makes it harder to get the killing blow on units.
2. Map Awareness, get used to checking your mini-map frequently and keeping mental tabs of where enemies are and where they are likely heading. This is something you will likely always be able to improve on even at the highest levels of play. Related to this is wards, always be sure to put a few wards nearby to make sure you don't suddenly find yourself outnumbered. If you don't know some spots to place them, ask a more experienced player or watch some pro matches to learn the more common spots
3. Learn the champions, while it would be mental suicide for most people to try and learn every champions ability at once, try to learn them on a match to match basis. If you're facing an enemy you've never seen before, be sure to note what abilities they have, including things like crowd control, dashes, ranged attacks, and shields. Try to carry on this knowledge to later matches and eventually you'll have most if not all of the abilities down. Of course this includes champions on your own team as well, always be sure to read the abilities of a champion when you are trying one out for the first time.
In summary, learn how to make gold, how to keep tabs on where friends and foes are located, and what champions are capable of what. These are the fundamentals of LoL and will be something you will improve on even at higher levels of play (though some less than others). Do not get overly concerned with special item builds or things team compositions till later on. Go with recommended set ups and items until you get a firm grasp of the basics, or optionally look up a guide if you want to try out something different.