Go up and receive your Owl Post from the Owl at the stand, by pressing B. It's a message from Ron(ald):
"Hi Harry, you'd better be careful in the shop. Don't let the miserable old shop wizard catch you or he'll throw you out. - Ron"
Point taken! Now go up and get a Knut from the chest, then enter the room. Harry will tell you to sneak around by holding the B Button; this isn't very compulsory though, but you can walk closer to the shopkeeper without getting caught. The shopkeeper is marching around the desks in a rectangular fashion and weaves in and out between the desks, so if you're running don't get within a third of the screen's distance close to him, or if you're sneaking don't get a third of the screen's distance facing him. Don't sneak too close to the Fire Crabs in the room either, for they will scorch you. Equip your Flipendo spell, and cast it on a switch on the northern wall, shaped like a shield and with a handprint. A door will slide open, allowing you to go further.
Drag the stone block onto the switch to open another door, then enter and blast away the two Orange Snails for a Cauldron Cake. Carry on, and you will come to a barrel in front of a switch. Stand behind it and Flipendo it onto the switch to drop a chest. Open it for a Sickle, then blow open the pot in the top left corner for a Knut. Then go right and flip the lever to open a door. Enter the room and smash Trevor the frog with a Flipendo spell to stun him, then pick him up! Smash the two pots in the room for a Pumpkin Pasty. Then backtrack into the previous room, and next to the barrel you will see a Flipendo switch (the shield- shaped one with a handprint). Activate it by casting Flipendo on it (like always), and from there go through the following room, avoid the shopkeeper and exit the Magical Menagerie.
Neville will "accidentally" drop his Chocolate Frog, then walk off. Catch Chocolate Frogs by stunning them with Flipendo, then you may walk over to pick them up. They add an extra half-zap to your maximum health, and there are five in total. Bearing this in mind, snag Neville's Chocolate Frog <CF: 1/30>. Somehow (hmm, the Frog must be like a fortune cookie!), you get a message from Ron telling you:
"Hi Harry, Chocolate Frogs are more than just tasty snacks. For every five you collect, you'll be awarded an extra zap to your stamina bar. Check in your Inventory to see how many you've collected so far. - Ron"
Now go downwards again and talk to the twins; they will tell you briefly about Gambol and Japes, the trick shop. Enter the building...
Gambol and Japes
...and talk to the shopkeeper, then buy some stuff; I suggest 2 items of each. The top item is a Dungbomb (they have 2), the middle a Stink Pellet (they have 4), and the bottom a Luminous Balloon (they have 4). When finished, exit and carry on downwards.
Go close to the girl at another building - once again, it's Ginny Weasley - and you will automatically follow her into a building, Flourish and Blotts bookshop.
Flourish and Blotts
Here, you will be introduced to one of Harry's boastful teachers: Gilderoy Lockhart. After a photo and a bragging session from Lockhart, as well as a brief confrontation with one of Harry's enemies Draco Malfoy and his snotty father Lucius, you will automatically exit the room.
Right, that's just about everything, ya? You've gotten all your supplies, now find Hagrid.