If you mean ARAM, then sure. It's a great place for you to learn about the kits of a lot of champs. You might find one you like. For summoners rift you might have a little work to do.
Step 1. Start developing your champ roster
Okay you are new and you'll need a fair roster of champs. I'm going to recommend you getting 3 champs per lane, 15 total because you don't know where you'll end up. If you don't know which lanes you want to play (and avoid jungle if you can until level 30) the go to one of the follow sites and try picking up champs that have a 50% win rate by 10 games played:
https://www.leagueofgraphs.com/champions/builds
https://lolalytics.com
http://na.op.gg/champion/statistics
For off role champs and just learning the game, simple champs are best. You don't have to spend much essence on them, as there are a lot of 450 and 1350 champ which are just fine. Don't buy champs with RP ever. You can filter the results by lane. Also, before you buy, try playing them a few times while they are in the free rotation. If you don't like them after a few games, look for another champ.
If there's a more expensive champ you are willing to save up for and you loved playing it while it was free, go ahead and save up.
Anyways, once you get a champ make sure you have a basic idea of what the skills are and which items to buy and when to buy them. Looking at the sites I linked to will tell you which are the best first 2-3 items to get on a champ. Buying a thornmail on Ashe in a real game will get you a lot of grief.
Step 2. Practice core basics
For summoners rift I'd suggest a 1v5 custom bot games against intermediate bots. Go into bot or top lane and practice CS for the first ten minutes, aiming for 100 while avoiding damage from the bots. Do that for an hour or two a day until you regularly hit 100. Then spend about 30 minutes in a custom 5v5 bot game practicing jungle routes, ganking lanes, covering lanes when the laner is gone, and trying to get a dragon. This is important because you might wind up in the jungle and you need to have a basic idea what to do.
The short version is kill the 2 buff monsters ASAP without dying or losing too much health, take one or two more camps to hit level 3, and then gank a lane.
While you are doing both CS and jungle practice, get in the habit of looking at the map every 5 seconds. This is super important for PvP because if an enemy laner is missing, he might be trying to gank you. While jungling learn to keep track of the time because the buff monsters respawn 5 minutes after being killed. As a laner you need to know this and watch the map because after one or both are dead, the enemy jungle is going to want to gank someone.
Also, don't forget to learn to group up. This is harder to do in custom bot games, but if you are doing Co-op, after about 12 minutes or so, start staying grouped with your teammates.
Step 3. Learn warding locations
Just google or search youtube for in depth locations. You know that stuff about watching the map? Wards give you extra vision. Grab one if you can afford it on your first back and remember to use the warding trinket.
Step 4. Learn to dodge games
If you've got a crap teams, don't worry it happens. Just hit that little X button in the top right corner and close the client. Problem solved, but you have to wait a few minutes until you can play again. Warning, once you get into ranked you can only do with once per day before you lose LP.
This step saves you a ton of grief in the long run.
Step 5. An important step
League is a game of minions. They help take objectives. They can literally win you the game. They can distract enemies away from what you are planning on doing. This is a pretty big topic but try to start learning about these guys and how to use them to win game. Learn things like wave control, wave freezing, and slow pushing for starters. This step is going to take you awhile considering all you have to learn, but they are the gateway to bigger picture play and planning ahead.
An easy example is if you've got a large wave of minions in a lane attacking an enemy tower, someone is going to show up to deal with it. That means only 4 other players on the other team at most will be available to respond. That means if you can set up a big wave, you can create an opportunity to take baron or a dragon.
