Early game/ laning - Into most matchups you'll want to play agro and try to zone them off cs so they can't outscale you, if you can play off your power spikes you can all in them and rack up kills early on, garen is great at punishing mistakes, never be afraid to punish the enemy if they mess up.
Late game/ grouping: If you have an engage support then great! wait for them to engage then go in and target their squishies, you'll either kill the squishy or have the squishy be focusing too much on you and not be in position to take out your backline, as garen you especially want to target ability heavy champs like lucian, syndra, xerath, aatrox, because of your silence. If you do not have an engage support then you might have to be the one to engage however your engage is not very good so in order to get close to the enemy you might have flank.