As you may already know, when you first start the game your first objective is to seek out the Great Deku Tree. Talk to Saria if you haven't already. If you try to take the trail directly to the Great Deku Tree you'll be stopped by Mido, who will tell you that you must acquire a sword and shield before you will be permitted to enter.
Head to the southwest portion of the map. When you look at the mini-map, you should see a few straight small trails; make your way towards them. Once you get there you'll notice a small hole in the ground. Walk up to it and hit the A button. Link will crawl under it. Dodge the rolling rocks and find the treasure chest at the back. There you will find the Kokiri Sword, which you are able to "borrow" for a while.
(Press Start and move to the Equipment Submenu to equip your new blade)
As for the Deku Shield, the only place to get one is at the shop, at the price of 40 Rupees. So head around town and start collecting Rupees. You can find some on the high up ledges, by breaking jars, by throwing rocks, by cutting grass, or even by simply walking around through the grass. Near the lake you’ll find three platforms with squares on them; each time you successfully jump across them all you get five Rupees. Just do it once, head into a building to have it reset, and then exit and do it again for five more Rupees.
Once completed, head to the Great Deku Tree, listen to his story, and start the first dungeon: Inside the Great Deku Tree.