Just keep playing and slowly gain experience and build your knowledge. If youre afraid of getting stomped, just play bot games for now. Roles wont matter as much and you can learn the basics.
At the beginning, you can just follow what the game tells you to buy. As you play more, youll learn what items are necessary for each champ. Read the descriptions and learn the passives and active of the items. Then when you have an understanding for most of the items, you will be able to tailor your build to counter your enemy. Theres isnt really and one size fits all build.
Learning champs is a lot easier now than it was back then. Upon death, you can use the dropdown to see the skills and damage used to kill you. Try to read those to get a brief understanding of the enemy champ if you dont know the champ at all. After the match, take some time to look at the champs in your profile and watch their skill videos.
Theyve also added a training room where you can give yourself tons of gold and experiment with champs and builds. Its a great tool to help you learn what works and what doesnt.