z' has provided the solution, in Vegetable Starch.
After having played through the first part of the game six or seven times, here are some tips on getting your Vegetable Starch production going.
You need three things for Vegetable Starch:
- At least one Corn, Tato, and Mutfruit
- Settlers to farm the crops
- Purified Water
You may happen across the seeds randomly as loot or being sold by a vendor. I've bought all three of those things from Trudy at the Drumlin Diner, but not all three at once in the same playthrough. I've found corn in a container as early as the Workhouse in Concord.
If you don't happen across the seeds, there are locations where you can always find them. Abernathy Farm has plenty of Tatos, but no Corn or Mutfruit. If you follow the usual path from Vault 111 to Diamond City, the first place you'll see all the crops you need is Graygarden. You can make a beeline here from Vault 111 with barely a combat encounter, if you want to get things jump started.
At Graygarden, you'll see many Mutfruit outside. Inside, you'll find Corn, Tatos, and Mutfruit planters. These refresh on their own even before you gain Graygarden's allegiance.
I suggest setting up a farm in Sanctuary.
You can put up a recruitment beacon before meeting Preston, if you like. You can find most or all of the components in Sanctuary and Vault 111, the most difficult being the Crystals.
Or, you can rescue Preston's group from Concord, and meet up with them in Sanctuary. This will give you several farmers immediately. You can assign anybody but Preston to farm, including Mama Murphy and Sturges.
You'll probably want to re-plant your first few harvests. You want at least three of each crop type, and probably six or more.
You'll want to manually harvest each crop whenever you return to Sanctuary. In addition, some crops will be auto-harvested and deposited in your workbench. It's not well understood how this auto-harvesting process works, but I can say from experience that it's okay to leave the crops in your workbench.
Since you will be planting more food than you need, I suggest you store all the melons already at Sanctuary. This reduces the defense you need to cover all your resources, and frees farmers to work on useful crops.
You can set up a water purifier in Sanctuary's stream before even leaving Sanctuary to go to Concord, and I recommend you do so. Add additional water purifiers over time, and sell the excess bottles to increase your income.
If you get Science I, you can build an industrial water purifier. This is great, but make sure you have enough defenses to cover all the resources you are producing.
Making the Starch
Now that you have all the components, you need a cooking station. There is already one built in Sanctuary, near the main bridge into town. Note that you can move this thing, even if you can't build one. I usually move it closer to the other workstations in the center of town, and plant my crops in the large grassy area there (near where the melons are by default).
When you make Vegetable Starch at the cooking station, it is put in your inventory. You'll want to store it in the workbench before leaving town.
After a while, you'll be producing more adhesive than you can use. This is okay. Making the adhesive gives you experience, and you can sell the excess, or just use it to craft things you don't need for more experience.