Garen is tanky at lvl 1 with one of the nicest passive in the game Perseverance which is free health regen. Garen has a silence which is always great in any team. He has a decisive ultimate where he brings righteous judgement.

Silence
Sustain Perseverance
Great Health
Great Armor
Escape ability
Built in tenacity

Melee
Low damage
squishy/slow
long cooldowns
team reliant

In my opinion go can use AD runes due to pretty high health and armor early game. Going solo shouldn't be a problem and has good escape ability.

Since you gonna be a tank go DEFENSE.

Boots 3x Health Potion is standard and is my preference for garen.
Heart of Gold i finding last hitting to be boring with Garen so i get this for free gold and 200 health and free health regen from passive.
mercury's threads magic resist for those annoying AP casters and debuffs.
Randuin's Omen great defensive item and has a great active that slows enemies for what ever reason.
b.f. sword i get this early-mid game cas with out it you'd be doing very little damage.
Vampiric Scepter life steal good for killing creeps and gaining health.
Bloodthirster after 40 kills could be your AD would be 100 better than Infinity Edge and some life steal.
Force of Nature must for tanks with a little movement speed.
Warmog's Armor Great health and regen, that the enemies will feel as if going against Baron Nasher.
Zeke's herald you should be very tanky by now and should get some type of aura item like Sunfire Aegis or anything that'll help your team mates.
P.S. DON'T GET ANY ITEM THAT GIVES YOU MANA IF YOU DO, YOU DON'T.

Last hit if possible. Try to use brush to your advantage. I recommend playing it to your tower since garen has to chase his enemies.

Heal FOR TANK HELPS YOUR TEAMMATES
Ghost I LIKE GHOST ON GAREN BECAUSE I FIND MYSELF NOT BEING ABLE TO FINISH MY KILLS AND GETS REALLY ANNOYING.
Flash NOT A BAD ITEM
Exhaust GOOD CHOICE TOO
Ignite NOT BAD FOR EARLY GAME KILLS

Garen is very tanky and not really squishy in my opinion. He just needs the team to do damage if you build him as a tank but feel free to get offensive iteams and stack kills rather than assists.