There are several different ways to get Riot Points (aka RP) to buy skins and champs in League of Legends. No matter how you do it though, you’re going to have to purchase them. You won’t get any free Riot Points unless someone gives you some, and good luck with that.
The simplest way to get some Riot Points if you don’t have a credit/debit card is to go to your local big mart gaming store (Best Buy, etc) and purchase a Riot Point card there. Once you have the card you can open the League of Legends client and click on the orange treasure chest at the top to open up the store. The store will have different options, but the option you want is the orange “Purchase RP” button in the top left.
If you bought an RP card you would select prepaid card, enter in the code from the back of the car,d and you now have RP to use for skins and champs. There are other options here as well if you have a credit/debit card, paypal, paysafe or a mobile payment. They have many options on how much RP you want, and will give bonus RP the more you spend.