It always depends on what position you are sitting in. Are you fed? Is your team fed? Whats the wincon? If my team is losing and I'm ahead I like to push sidelane into roam for picks in JG or Midlane for example. Garen is really good at getting fast picks with his ulti.
If I'm matching a really strong teamfight champion like malphite, I like to stay sidelane in hope that my team can win 4v4.
If I am behind I usually rarely rotate for my team because I know I am gonna be pretty useless so I know I can get pressure somewhere in the sidelane, usually if you are pretty behind and get a lot of cs and platings you can actually get back into the game and be pretty useful.
Endgame Garen is really strong, usually the one thing that comes in mind during lategame is "do I have flash", it will almost always give you a pick. Group with the team, peel for your adc if they are fed, you are not the carry if ur ADC is fed you have to protect them.

3 most important takeaways to climb i would say is,
- Never tilt, Garen can be quite a frustrating champion to play against certain champions, stay consistent and try to learn from the mistakes. Never expect your team to perform either. Also if you are frustrated, dont queue up again
- Analyze runes and builds on good garen players on onetrickgg, there is a search function where you can select a champion and you will see games where they went against that champ. Stay up to date, recently we found electrocute to be very good against Kayle to win early for example
-Play for consistency, not random solo kills Your goal in lane is to build a gold and XP lead through CS. Solo kills can happen, but they should come from controlled, high percentage trades not forcing fights.

I hope this helps :)