You begin the game in Diagon Alley. Hagrid will take you to the bank and tell you to get your money to buy school supplies. Go inside, talk to the goblin, then enter the door to the North.
The track is in need of maintenence, so you have to go the rest of the way on foot. Open doors, flip switches, and kill Fire Crabs with your Flipendo spell. There's a part where you need to collect five Gem stones so you can get the Incendio spell. To get the stones, try pushing some mine carts, opening treasure chests, crushing boulders, and other various methods. After collecting the stones, go to the door that has the Incendio Spell Book behind it. There will be a Giant Fire Crab in there. Just charge your Flipendo spell, and aim it at him. After his life meter is empty, the Fire Crab will burst into flames and leave the Incendio Spell Book behind. Pick it up, and you'll have the ability to create Fire!!!! Go to your vault, and cast Incendio on the big cobweb. Collect your money, then Harry will automatically take a mine cart back to the surface.
Now that you're back in Diagon Alley, go to the otion Shop and buy 2 Potions sets (one for you, and one for Ginny). Next, go talk to Neville Longbottom. He'll tell you that he got kicked out of a shop, and that his pet toad is still in there. Go inside (don't let the owner catch you), and solve the puzzle (don't worry, it's very simple, *rambles about how he sucks at Zelda games*). To catch the toad, cast a charged Flipendo spell to knock him out, then walk upo and pick him up. Now leave the shop, then talk to Neville. With Neville's toad in safe hands, a Chocolate Frog in your belly, and Ginny waiting at Flourish and Blotts, go to the Joke Shop to buy some supplies (Dungbombs are good against Trolls, I'm not sure what balloons are for, and Stink Pellets are self explanatory).
Poised to play pranks on your foes, go to Flourish and Blotts, talk to Ginny, and get your picture taken with Gilderoy Lockhart.
WALKTHROUGH ENDS HERE!