I play Garen a lot and unless they buff him in some respect, your never going to be a powerhouse in the late game like you are at the start and mid.
What I do with Garen my building order is Dorans Blade, boots(Ninja Tabi or Mercs depending on the other team), Brutilzer and then depending on how your doing you can finish up your ghost blade, or get a giants belt and start building your defenses up...
Giants belt turns into ether Warmogs or a sunfire cape, if the team is heavy on AD then get a sunfire, AP then you go warmogs. And if/when the game gets into its late phase and your attacks aren't doing much, this is where you can make the difference between being useless, and being a huge asset, and you figure out what you can do by asking yourself these questions.
1. Does their team have a tank, or a very defense heavy player?
2. Who is the most venerable to being silenced?
3. Can I run into their team, and live long enough or escape so my team can attack without fear of being killed?
4. If I find myself unable to do any of the other things, am I better off building support items?
Garen has one of the best ults in the game, for the sole reason that its like playing with all your enemy's having 20-25% less health, once you get them that low its a done deal, what also makes it so great is that it does base magic damage plus extra for all the health they are missing, which means in a late game fighting an Alistar who has 3.5k health, even using your ult when he is at 50% will do a crippling amount of damage which will allow your team to kill him that much faster and get him off the field, and the same goes for any tanking champion or bruiser.
When used correctly, your silence can mean the difference between the enemy's Annie getting off all her spells and Tibbers to destroy your team, and your whole team killing her in the blink of an eye with all their spells at once. Yes its that important and the great thing is, if you use ghost blade with Decisive Strike will allow you to catch even a fleeing Yi.
Garen is not a tank but until his damage is buffed for late game fights, your only role in most team fights comes down to you running in and doing your best to stay alive while your team comes in and starts taking them out while their spells are focused on you, hence why you want a couple tanking items in your build even if your a full AD Garen.
Sometimes even with your best setup and a good team, the other team might just be to smart in not letting you ult their tank, silence their carry's, or they ignore you and kill your carrys instead of wasting attacks on you, those games happen. The best thing you can do at that point is play a more supporting role by getting Starks Fervor, and a Legions, both of which make a huge huge difference to your team. You might give your AD carry that little bit of extra lifesteal he needed to kill that Morde that was stomping your team before. At this point all you can do is stay alive, and do damage where you can, every little bit helps. Also get an Oracles elixir and do a bit of ward killing, every little thing only helps your team.