Navi the fairy wakes Link up and asks him to visit the Deku Tree. He must get equipped before Mido will let him pass.
After Navi wakes you, go outside and watch the scene. In the 3DS version, you can press Start to go to the save screen. You can also change some options from this screen.
Go down the ladder and talk to Saria.
Before you can visit the Deku Tree, you will need to acquire a sword and shield. There is a sword hidden in the forest that you can get for free, but you will need to have 40 rupees to buy a shield.
Get a Sword
Walk to the end of the path leading to your house, then turn left. Walk along the path and you will see a slope on the left. Go up it. Go through the fence maze (or just climb over the fences) and crawl into the hole in the wall. Press A to crawl into the hole, then go through the crawlspace to the other side.
Turn left and look for the rolling boulder. When it passes, run and follow behind it, and when there is a gap in the wall to the right, go in to get a blue rupee. Turn around and go back into the path that the boulder is rolling around. Go straight, then when you see a gap on the right, go in and you will see a big chest. Open it to find the Kokiri Sword. Be sure to equip it. In the 3DS version, you can tap the Gear tab in the touch screen and select the sword to equip it. In the N64 version, press start to bring up the menu, then press Z or R to bring up the Equipment screen, and equip the sword from there.
Now turn around and watch for the boulder again. When you see it, follow behind it again. When you turn the corner and the boulder is out of the way, you'll see a blue rupee next to a bush. Get it. You can use the sword to cut the bush, which might have a rupee in it. Crawl back out through the hole that you came in through.
Get a Shield
Now you need to get more rupees. There are several blue rupees scattered throughout the forest. You can also cut bushes or run through tall grass to find green rupees. The shield costs 40 rupees.
In the fence maze, there are a couple of signposts. Z-Target the farthest rectanglar sign (read it for an explanation of how to cut the sign down) and, while holding forward on the control stick, swing your sword. You should execute a stab, which will take the whole sign down. You will get a blue rupee for cutting it down.
Now go down the slope that you walked up earlier to reach the fence maze, and go into the house straight ahead. Open every chest to get a total of 11 rupees, and a recovery heart. Exit the house, and climb up the ledge behind the house (just walk up against the ledge until you climb up) and walk in the narrow passage here to find another blue rupee.
Drop back down. In the water near the house you were just in, there are three stepping stones. Jump across each one (when you run over an edge, you jump automatically) and when you reach the last one, you get a blue rupee. Now you're near the shop. If you don't have 40 rupees yet, you can go into the shop (you will have to talk to the girl above the door first) and come back out again, then jump over the stepping stones again. If you still don't have 40 rupees, just keep going into houses and jumping over the stepping stones until you have enough.
In the shop, talk to the shopkeeper, and buy the deku shield. Equip it right away, then exit the shop.
Go to the Deku Tree
Outside the shop, there is some water on the left. Go across it and go toward the arch on the left. Talk to the guy who is blocking the arch. Mido will let you pass if you have a sword and shield.
Follow the path and eventually some Deku Babas will come out of the ground. Z-target and swing your sword to cut them down. They leave behind Deku Sticks. You'll need these later, so be sure to collect them. You can carry up to 10 Deku Sticks, and the Deku Babas will keep reappearing after you kill them, so feel free to keep collecting Deku Sticks until you have 10 of them.
After clearing the Deku Babas out of the way, continue on the path toward the Deku Tree. A cut scene will start. Say Yes when he asks if you have the courage to help break the curse, then go into the Tree.