First, you might like to visit the church and save your game. If you're using a password be sure you wrote it down correctly, as a single incorrect letter will render the whole password unusable.
Getting the first heart container
Head to the far left, jump over the Jump Block and enter the bottom door. Head to the right and down the hole, then to the right. Open the chest to claim the first 
Heart Container. Now make your way back outside. See that locked door? You need to get in there, but you have to find the key first.
Finding the key
Head back to the right and enter the bottom door in the building to the left of the church. Jump down the well, then head right and up the stairs. Continue to the right and jump across the platforms. You're now on the beach. You need to duck to hit crabs. Don't go in the ocean just yet if you can help it, as the things down there are harder to do in your current state. When you reach a little house ignore it and keep going right. Eventually you will reach a sort of brown hill. A cloud will appear and start dropping bombs. If you have the 
Arrow or 
Thunder magic you can blast it out of the sky; otherwise just avoid it and head through the door to the far right. Go through the right-hand door and open the chest for a 
Key. Now head back to the town.
Getting to the pyramid
Back in the town, open the door with the key (use the up arrow block to jump up there) and head to the right, then through the door. Now head left. After a while you'll reach a pyramid. Fall into first well then once inside go through the door to the left. Once through that continue left and up the pyramid. When you reach the Sphinx, slay the enemy there then go in to get your third Heart Container. Go back outside. Watch out, the enemy has now respawned. Go back up the pyramid and this time fall into the well to the right. Head left until you're inside the pyramid itself. Make your way to the top and get the key. Use the key on the locked door.
Head left over the floor made of springs (watch out for enemies beneath you). After this go down the hole. You'll end up stuck between two fire heads. Quickly go to the left or right and you'll fall down another hole. Go through the door in the center.
Second boss: Mummy Dragon
You now fight the Mummy Dragon. This is a fairly straightforward battle. When the dragon comes towards you, jump and shoot fire in its face. If it's too high duck instead and use the Arrow. It will frequently spit green slime. If you stay directly beneath it, this attack can't hit you, but then get out of the way as it will usuall swoop downwards straight after. As with Meka, dodge the flame when the boss is dead and grab all the loot you can. When the flame touches you you will become Mouse-Man.