When Garen solo top lane the best way you can do is build tanky as much as you can. When building Garen tanky this Guarantees you to put more pressure on the enemy team and tank more damage on the enemy.

During teamfights what you want to do is immediately rush into their adc, provided with your Q movement speed and your tankiness, there nothing they can do. And remember when they're at you they're not at your team. Always initiate first unless your jungler is a better initiator. But remember its always situational, its up to you to decide whats best for your team.