You should just take your time and go through each of the champions on the "champions" tab on your profile. Just click through the champs and see what all of them will do. It will help you understand the game a lot more as you get an idea of what enemy champions can do, while also what ally champions can do. Here, you can also see what champions that catch your eyes. if you think a champion looks cool or his/her abilities looks cool to you then wait for them to be on free week or save up IP to buy them.

One other thing that you need to understand early in the game is the meta. But before I talk about the meta, I'll talk about roles. Every champion is classified into one or two roles. Tanks are very beefy people that will take most of the damage and normally has a lot of stuns or roots to lock down enemies. Bruisers/Fighters are a mix between damage and tankyness they deal a moderate amount of damage while also having some health to them aswell. Another role is assassin. Their objective is to flank in, use their combo onto someone, kill them, then get out. Note that they are very weak without any abilities and will die very fast if they are caught out of position. The next role is mage. Most of the time they are ranged casters that rely on ranged to get their damage off. They can have either sustained damage over time or burst. Their weakness is having very low mobility as they normally have no escapes. Another role is marksman, but is normally refereed to as ADC. The ADC is normally very weak early but in the late game they will be the main source of damage from your team. Some games it may turn out to be whoever can keep their ADC alive decides who wins the game. The final role is support. This role helps the team and will normally not get much champion kills or minion kills. They are either a tank support or a mage support.

In terms of the meta, its 1 person top lane, 1 person in the jungle, 1 person mid lane, and 2 people in the bottom lane. The person in the top lane will normally be a tank or a bruiser/fighter (same thing, different names) although in some cases you can play a mage or assassin in the top lane. I would not worry too much about jungling when you are first leveling up because the jungle is a pretty complex role to play at first sight and also you wont have the proper runes and masteries. If you do decied to jungle, you'll find that tanks and bruisers will work best. In the mid lane, you'll find mostly mages and assassins. In the bottom lane you'll find two people, one is an ADC and another is a support. The ADC's job in lane is to get as much farm or minion kills as possible. The supports job is to keep the ADC alive in laning phase as they are weak, and also during the late game as assassins want to jump in on them and stop them from getting DPS off. For a new player I would recommend playing top lane. There is a big champion pool top lane so you can play a lot of champions up top, just don't think you can play any champion in the top lane though.

Again I'm going to say this one more time because how important I think it is for a new player, just go into the champions tab and read what the champions do and you'll have a much better understanding of the game.