The rifle is an air-rifle. It takes time to pump them, and it probably needs a tool back at the house. So, you get one shot on every outing.
For fishing, us the rifle on whatever attacks you, and finish it off with standard attacks, not heavy. You'll have plenty of stamina left to fish.
After a certain point it will tell you to hide and surprise Robin. Go to the kitchen and select the hide option. After that, you'll have the idea to get fish to lure her out. Go to the pier, catch fish, go to your room, get the cookbook, go to the kitchen, follow the directions in the book and make the fish.
Seriously, the log and dialogue are pretty clear about what you need to do. If you somehow get lost, just check the glowy things around the houses rooms.
Just make sure to equip your weapons as you get them, otherwise you can't use them.
Another tip, anytime you need to make something, you seem to do it in the kitchen at the table. That's where you repair the axe, make the spear, and so forth.