Juggernauts are actually one of the harder champion types to play effectively as a new player because most of the challenge is getting a good engagement that lets you actually use your strengths to win fights and/or get objectives, which itself may be heavily composition-dependent. This requires good macro and game sense rather than champion skill, which are typically harder to develop.
As a new player your team is too dumb to play effectively around splitpushing and you're probably not great at warding and reading the map effectively to know when you are in danger, TP management, and finding flanks. You will have a lot of fights where you are being kited around by Ashe, Jinx, or Jhin and die slowly but without getting much done.
Basically, you can definitely one-trick Garen and win lane a lot, but your goal is to win the game, not the lane. There are compositions where picking Garen might actually just lose you the game in champion select and you may not be able to identify why you lost.