Adventuron's advice is very good advice. Read it carefully.
Make the objective fairly clear right from the start. Allow the story to develop gradually, just like a good book, but don't forget that this is a game, not an ebook. It should be puzzle-based with short descriptions and it should be fun to play.
Think about where it is set (what country and culture, city or village, indoors or outdoors). Also think about when it will take place (ancient, medieval, modern, future). And make sure that everything is in theme.
We know that there will be a spooky house and a graveyard, because they're in the rules, but is it a Gothic mansion, a dilapidated house in the country or a condemned apartment building in the city? What sort of rooms would you expect to find in that house? How are they connected together? (Mine is set in a Victorian-style house and I actually did a Google search for floor plans of Victorian-style houses so that it had the right "feel".) What sort of objects would you expect to find in those rooms? For example, you wouldn't have a bedroom without a bed, but what style of bed? Draw up a list of what objects you'd expect to find in each room, then think about how to incorporate those objects into puzzles? Some objects should be easy to find. Some objects should be hard to find. For those objects required to solve puzzles, some objects should be found before you encounter the puzzle. In other cases, you should encounter the puzzle before you find the object needed to solve the puzzle. In other words, mix it up, don't make it too easy or predictable.
Try to use ordinary objects in unusual ways. For example, a broom is normally used to sweep a floor, but it can also be used to get something that is out of reach or to defend yourself against a foe or to repair the rung of a broken ladder or to play a game of Quidditch.
As your puzzles start to develop, you will find the need for more objects. Where should these be located? Should they be hidden or in plain sight? Again, mix it up.
Once you have worked out all your puzzles, discard most of the objects that you didn't need, but leave a few as red herrings or to give a sense of realism.
Adventuron said that EXAMINE is not a puzzle and he's right. EXAMINE should be used to gradually reveal more about the game, reveal hints and (depending on your theme) add an element of humour. Virtually every noun should be examinable. Give short room descriptions with very few nouns and gradually reveal more information as you examine all those nouns, but don't make it boring or repetitive. As a rule of thumb, avoid digging down deeper than three levels. And don't be scared to say "You say nothing special" if there's nothing special.
Give careful thought to the vocabulary. Add every verb that someone would be likely to use, even if they're not needed to solve the game. Then add some verbs that people are not likely to use, but if they chance upon them, they'll appreciate that you've given a sensible response.
'Mushroom Hunt' had beautiful graphics and beautiful descriptions, but it had spelling errors and bugs, virtually no puzzles, limited vocabulary, some very unfair scenarios and was very tedious to play. This is not a good example of an adventure, so don't use it as a model.
A much better example is 'The Troll's Revenge'. This had its faults, but it was not boring or repetitive and had some really nice puzzles. The apple cider puzzle was probably one of the best in the CaveJam, because it had multiple elements required to solve several mini-puzzles. Another good example of this was 'Seeker of Magic'. It had a lot of puzzles packed into a few rooms and nearly all the puzzles had multiple elements. For example, you needed the knife to cut the apple to get the worm to give the troll to enter the cave to get the rock to disable the troll to get the pouch to get the flint to light the torch to get into the dark cave. And there were lots of hints provided along the way.
I hope this helps and gives you some ideas.