Okay.
Go to screen 76 in the file in ZQuest and click on the Screen Menu.
To change what the cave does, you will want to change the room type. You have a few options here: Guy, String, Room Type, and 'other' (which changes based on room type).
Click Room Type and select 'Shop'. Be sure you don't pick potion shop, otherwise you will need the letter to talk to the guy. Then return to the Screen menu. You'll see the 'other' option has changed to shop type. Click it. A window will pop up with a list of different shops. They generally have names, like Bomb/Bait/Heart M/X/C. These names are traditional (it means the shop has a bomb, a piece of bait, and a heart, and that the bomb costs a medium amount, the bait is expensive for the game, and the heart is cheap), but if you make your own shop type you can name it whatever you please.
Pick the shop you want and then click okay. Finally, you'll want to change the message the guy says. This is the screen's String. In the Screen menu, click String and select a new message for the guy to say.
Let's take a little aside to discuss custom shop types and custom strings. You can make new messages and shops in the Quest menu. Let's discuss Strings first.
To make a custom String, go to the Quest menu and select 'String'. Scroll down on the list until you see <New String>, click it, and edit it. If you leave the attributes alone, it's essentially just a text box where you can type your message (which will display as it appears in the editor). If you adjust the width or height of the string box, though (in the attributes tab), I can't guarantee that the message will appear in game the way you intend.
Now, let's discuss custom shop types. Again in the Quest menu, go to the Misc Data sub-menu and select Shop Types. A list will appear. Scroll down to an empty shop and click edit. You will be able to assign up to three items, as well as prices for each. Be careful! You should pick items top to bottom; the game will center them automatically. That is, don't leave the first pull-down 'none' if you want the item to appear in the middle; if the shop has only one item then it will automatically appear in the center. Be sure to give the shop a name that you can identify, then go back to "Screen->Shop Type" and change the shop to the one you just created.
Okay! Finally, let's discuss Enemies. Enemies can also be set through the Screen menu. Go to Enemies to open a list. To place an enemy, double-click on the 'empty' slot or on an enemy that is already on screen and then select a new enemy from the list that opens. If you wish to copy an enemy, press the 'c' key and then click on where you wish to place them and press the 'v' key. If you wish to delete an enemy, click on it and press the 'delete' key.
Two other enemy pointers:
1.) Just like with shops, you shouldn't have the first enemy be 'none' or nothing will spawn on screen. Any enemy that is listed after a 'none' enemy also will not spawn, avoid leaving gaps.
2.) Enemies are spawned in the order on the list. When you defeat an enemy and leave the screen, the screen's number of enemies is reduced, but the enemy defeated is not recorded (this is just like Zelda 1). What I mean is this: If you place two octoroks and a darknut on the screen, kill one of the octoroks, and then leave the screen, when you return there will only be two octoroks (They're at the top of the enemy list for the screen, and the screen records that there are two enemies left, so only the octoroks spawn). Experiment with this a little; sometimes if you want a screen to be difficult you put the strong enemies at the top of the list. If you want players to have a break, then the weaker enemies can go on top and the stronger enemies lower, so that leaving and returning makes the room easier. Finally, bubbles are generally placed at the top of the list as good practice (though you don't HAVE to do that!).
Always glad to help!