Unlike other hm games the rucksack in this game has three different sections. However you don't want to keep things like crops and food in your rucksack for too long becuase they will expire.  For a longer life for your produce and food dishes you should place them into the horse cart. 

To access your bag all you have to do is click either of the three icons on the left.  The first one is the Bag, second one is fish and third is insects. 

Once you're inside your bag you can move to the three sections within it by clicking on the really tiny icons at the top of the bottom DS screen.  We circled them in red as well as marked each section with the icon. 

The first sections is Bag where you can see items like tools and equipment like seeds and what not as well as things you’ve gathered in the mountains or harvested crops. 

 Then there is the fish section where you can find all the fish  you’ve  caught.  You can access it by the little blue circle in the    bag category on the screen.

 After fish comes the critter screen where you can see all the bugs you’ve caught lately. 

* Pretty much everything besides your main tools can be sold when you open up your bag at the shipping bin.