i use a double pallisade wall around my base in the start. its worth the extra wood harvesting. snap a floor piece on the upper part of the pallisade and you can make a wall you can walk around and shoot down from.

To keep produce safe, a neat trick is to build your farm at a remote place you don't visit often. Since Valheim doesn't actively calculate anything in areas not visited by the player, the crops will stay completely unharmed. Wait for 3 ingame days after planting (remember onions take a day longer to grow!), then come back for harvest. You can even see the crops being updated from "freshly planted" to "fully grown" if you're fast enough.

You still might want to put a wooden fence around them to add a bit of safety when you're around for harvesting.

It is a good idea to box your entire farm with walls and work benches. The wall will keep random patrols from charging through the fields, and the work benches will prevent spawns inside the farm.

The general farming process is to gather wild seeds, plant them to grow food crops, and then plant the food crops to generate more seeds. You may have nothing to eat for several cycles until the farm is big enough to take off.

take seeds and plant them at home. You can replant the product for more seeds

Start looking for another swamp.

Once turnip seeds are picked, that plant does not regrow.

It might be a good idea to always save some seed.