The Docks are accessed through a Keeper glyph. I'm not sure you are "allowed" to go there yet. If I recall, that access comes a little later on in the game.
Have you found the fences in the part of town where Garrett lives? They will buy and sell most of the things you need right now. If you have trouble finding them, the areas they're in always have a little red "Hand" symbol, and near Garrett's place, this is located on a wall behind the little guard station where you can find arrows.
Keep them mowing blades sharp...