I think, if you have health runes, you can start with Boots + 3 health pots. Otherwise get Doran's shield.
For your first skill, start with Q (silence) if you have an offensive laning partner (ie: tryndamere, caitlyn, ashe, etc). That way, they cannot use their summoner's spell to escape. If you have a CC laning partner, start with E instead, such as Taric or Mundo's cleaver to slow. Hide in the bushes and surprise people
Then get swift boots. The other items are situational, but usually items that give Crit, such as Avarice blade > Youmuu's, Infinity edge, are good for DPS. Sunfire cape is also good as it works very well with your E ability.
Level the abilities in this order, R > E > Q > W. I usually alternate E and Q, with one point in W before level 6. Usually, with swift boots you can run in with Q (for the faster run speed), use W to have higher defense, and spin with E. When they are low on life, use your R for the finishing blow. Towards the end, it can kill almost anyone with 1/3 of life left (depending on situation of course )
I'm not a pro-garen user but these are what I've learned so far. Any feedback appreciated too
EDIT: By the way, Doran's shield plus your passive allows you to regen HP safely if you run back to the turret, so you can stay there longer.