My client bugged out and wont let me ban teemo like i usually do. What do you know i get a teemo in my game and he abuses the lane hard. His damage output in season 8 is a lot higher than season 7 so it's harder to fight back against it plus his movement speed bonus. Any advice from garen mains or just any player whos experienced this match up? I used to rush mercury treds but even that isn't enough anymore.
I would start Doran's shield and rush a Spectre's Cowl, maybe take Second Wind too. If he's aggressive and keeps autoing you, move back into your minions so that he takes minion aggro. Just try to make it a farming lane and don't take any risks in fighting him unless you can sigh garen-tee a kill
well as garen there are a few things you can do in rune setup to make it easier.
go resolve primary with grasp , bone plating, second wind, and revitalize so when you get dorans shield + spectral cowl rushed you heal alot more from damage. also you can get null orb from sorcery to get a magic shield when low hp to stop from dying as easy . and celerity to move faster
your 1st buys unless stomping somehow should be 
into 
now you are asking why ninja tabs? well teemo auto attacks and does some physical damage and ninja tabs gives you armor vs autos/physical damage and reduced damage from autos (blocks 12% damage from autos ) now if you are vs a team with HEAVY CC dont get ninja tabs just cuz of teemo go ahead with mercs.
from those items you can branch out into other items needed for game, BC,TRI, HEX into MAW, Steraks etc etc also max Q 1st and you can take W 2nd to start getting more bonus armor mr early.
pink ward at teemos lvl 6 so you can clear for shrooms or get red trinket early.
This isn't exactly best matchup for teemo. After level 6, if Garen has ignite he can always 100-0 Teemo with flash QERignite combo (you need to wait after he uses Q on minion or on you before doing this). But if you don't want to risk this then all you need to do is to not feed and then do more in teamfights. I would recommend go minion dematerializer if you struggle csing against Teemo.
And also get 
and try to buy 
to replace usual yellow trinket.
Adaptive Helm.
quite honestly, just follow the build suggestions above, but go all in on him. Too many people make the mistake of letting him poke you down. Teemo is easy to bully out of lane if you jump in fast. Just make sure you are not hitting shrooms and you should be fine with Garen.
Rush into 
Once you get these, Teemo's laning power is null. He legit won't be able to kill you.
2 ways to handle this. If you have ajungler hit by teemos global passive, play safe, farm with E (and max E for this purpose, 2 lvls in q for the run time). Start dorans, and dont take unnecessary poke. ABUSE THE BUSHES.
When your jg comes to gank, he most likely dies. Build adaptive helm, and laugh at his damage. You outscale increadibly, and once you get a damage item, will spin him to death. Unless ahead, he cant push into you post 11
Second way, take predator, and ignite. Start boots and 4 pots. Trade agressively. Hope that jg is interested in teemo. Otherwise look for a chance to all in either after first back, whenevver he is low, or when you hit 6 (hp allowing). When you all in, predator and Q when closing (rocket like move speed), he will blind, DO NOT HIT HIM, start spinning (goes through blind), if you can, w his blind, when spin runs out, you wont be blind, q goes off, and you can ulti (ignite if needed).
if you go full ad garen i promise you will fuck him so hard he will either ask for a camp 1v2 or feed his ass off and actually ragequit and quitting teemo forever after that game. here are some examples with their orders :
-- > 
(these 2 can be in whichever order you prefer) , 
or 
, --> 
--> 
boots should be your second item and you can go either 
or 
you have to prioritize staying in brushes, your combo once you get out is QER ( proc + electrocute + 
[optional for early game secure if he manages to survive your R and lane dominance, unnecessary late game as you can one shot without it] will nuke that teemo in 0.2 seconds)
A MORE IN-DEPTH TUTORIAL ON HOW TO PLAY AD GAREN VS TEEMO :
