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how can i make a system like monster hunter, like where you need certain items inorder to make something? 

You can do this pretty easily with forks. I'm unfamiliar with Monster Hunter, but this would be a general item creation sort of event:
Fork if IngredientItem#1 have
-Fork if IngredientItem#2 have
--Message: Make a ResultItem with RecipeItem#1 and RecipeItem#2?
--Choice Yes/No
---Choice Yes
----Remove RecipeItem#1
----Remove RecipeItem#2
----Add ResultItem

That's pretty generic, as well as fairly simple to follow. If you don't get this, then I guess I could take a screenshot of an actual event that functions like this. Hopefully this will suffice. If this isn't what Monster Hunter II's system resembled, then describe further what you want.

Shouldn't you use conditional branches, because the player may not have recipeitem#1 for example.

Sorry, I'm using the RM2K syntax. The RM2K3 "Conditional Branch" is named "Fork" in RM2K. Just what I'm used to. The forks at the very beginning check to see if the player is carrying the correct recipe items.

Well, thats solved.

i'll try, but i have another question, actually 3

1. how can i make it so you can only have a limited inventory space?
2. how can i make it so you can place any items you have in a chest of some sort
3. how can i make it so when you kill a monster, you can scrape there bodys for items, i know i have to use variables, but i dont know how.

Could you be clearer in 2, you mean like a storage?

1. This is not a fun thing to do in RM2K/3. It isn't possible without custom shops. You must tally all items held, meaning having a command for every item in your game. Then, if this is equal to the cap, tell the player their inventory is full. Shops and monster drops can't be used; you'll have to implement these yourself to do the full inventory check beforehand.
2. I've had to work a double-party system that involved item storage before... Without scripting, it seems to resorts to having a whole chain of tedious forks for each and every item, as RM2K/3 doesn't allow to fork to see if a variable item is held. No references allowed there... It's one of the things that makes these makers a nightmare... Luckily you can add the item simply with a loop and array index. It's the checking the inventory that's difficult. I wouldn't recommend this without a lot of time on your hands. It's simply not worth it.
3. This is provided in the game by default. Simply check off the "item" under the monster section of the database. You also have the ability to set the probability that the item will be found. If you wish to add more than one item... You need to have fake monster deaths, where if they reach a certain HP%, kill them off and execute the body-searching stuff. This isn't fun. I've quit a game after applying this to half of the monsters and giving up.