For this you will need to have access to the farm. Keep expanding and checking the inn to find Aubrey who will eventually show up and grant access. It's found to the left as you go up the hill.
Aubrey will sell seeds for vegetables and fruit trees. They are seasonal, meaning they will only produce in the correct season. It's quite clear when you buy them. Vegetable seeds are wasted if you plant them in the wrong month. Fruit trees will grow but will only bear fruit in the relevant season(s).
Recipe requests come from Oma at the inn. Requests sometime appear on the board outside your home (if it is unlocked) or from Oma herself, so if nothing it showing up try talking to her. They appear to occur roughly every 3-4 days.
To get a multi ingredient recipe request, I just needed to complete regular, single ingredient requests. I did 5 and my 6th involved 3 ingredients.
I
believe that all requests involve ingredients that are available in the current season and all give you ample time to grow the ingredients from scratch. I spent a lot of time stockpiling heaps of fruit and vegetables but the more efficient method would be to just grow what you need for Oma each time you get a request.
Remember that you will also want to grow one of every vegetable for
Vegetable Farmer, so each season don't forget to grow one of everything for that season in addition to the recipe requests.