From my experience, the biggest headaches were:
1. Soil and plant compatibility. If a soil is too clayish or chalky seeds just won't germinate. Do a soil analysis and compare it to the plant. Also, plant the seed in soil and keep inside before replanting outside. Makes it easier for the plant.
2. Correct sun and shade. Check the packaging for correct amounts lighting and distribute plants accordingly.
3. Freezing. Where I live it could freeze well into May. Cover new plants at night until it's safe.
4. Watering and drainage. Check the downsloping for good drainage. You might have to create artificial drainage so the roots don't rot.
5. Fertilizing. Use sparingly and in correct amounts.
6. Insects, worms, birds, other. The "other" includes squirrels, foxes, coyotes, etc. I use a sheer, see-through onion skin cloth over the plants.
7. Weeding. A time-consuming job but absolutely necessary as weeds will steal water and soil nutrients and crowd out the fruits and veggies.
Be sure to thoroughly till the soil if planting large amounts. Otherwise individually loosen the soil for each plant. Maintain proper spacing between each plant. Check the packaging. Tomato plants will need to be supported by wire cages or wooden sticks.
You can build a fence by using branches off trees and chicken wire. This keeps out the 4-legged varmints. Keep it as organic as possible, which is why fertilizers should be used with caution. If there are no bees in your area for pollinating, there are brightly-colored devices that will attract bees.
Late spring hail is a big problem here. Sometimes it's predicted but usually not. Learn to recognize hail-producing clouds. I spread canvas tarps over the plants. An entire season of growth can be destroyed in five minutes.
One time a tornado passed through. It just picked up my tarps and ate 'em up.