So this is just a quick guide for anyone who needs a little help solo topping as Garen, who is by far one of the easiest champions to play. Start slightly defensive and let them push to your turret until level 6 because your ult will be your way to kill one and then get out fast before you die as well.
This build works well even if you don't get a kill for some time since the items boost your damage and defense for the most part.

Engage with Decisive strike for the silence and speed boost, activate courage for the extra armor and then spin to dish out some serious damage to all the enemies in the area.

Note* Ravenous hydra does work with decisive strike but not with judgement.

The attack damage with your marks is a great healp early on.

The armor from your seals will help your overall survivability.

The magic resist from your glyphs will help your overall survivability.

The lifesteal from your quintessences will help with your overall survivability.

Building heavy offense with slight defense will make you strong early and with your runes, you will have a fair bit of armor to back you up so don't forget to take advantage of slight opportunities.

Pros
-Strong early game.
-If fed will carry your team no matter who on the other team is fed
-High enough armor to absorb alot of damage.
-Slow will make it easier to chase enemies for your team.

Cons
-Only about 3k health.
-Slow attack speed but with high damage output.