Rush tabis, get bramblevest into either cleaver or Steraks, play save till ~lvl 11 and from there you should start to outscale if he's not fed.
With garen, it's important to keep in mind the scaleing nature of your W passive and your E in mind and play around their spikes. At lvl 4, 7, 10, 13 and 16 his E gets another spin, so at lvl 11 you have 3 aditional spins and your ult rank 2 which is overall a big spike, and by then you usually also got your W stacked up.
Also, it often helps to get an early second point into W, E doesn't gain much damage from ranks till you have some more spins from levels and some ad to back it up, and Q doesn't cut it vs renekton, so makeing sure your W is up whenever he goes agressive helps alot at surviveing the early.
Also, if he dashes towards you, hit him with Q asap, if he's to slow on the W you can deny his stun and get away, else you atleast make sure he can't easily follow with Q. Also, if you manage to use E befor the stun, the spin will keep going while stunned, which isn't that relevant early as you hardly will cut through his sustain anyway, but it can basically deny the stun by still dealing damage over it's duration which is valuable later on once you scaled a bit.