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There can be several reasons for blossom drop on plants, including temperature fluctuations, watering issues, insects, disease, pollination, and nitrogen levels to high.

Here are some links to help you.

https://www.gardeningknowhow.com/edible/vegetables/tomato/growing-temp-for-tomatoes.htm

https://www.gardeningknowhow.com/plant-problems/environmental/why-healthy-blooms-fall.htm

https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-diseases.htm
https://www.gardeningknowhow.com/edible/vegetables/tomato/growing-tomatoes-guide.htm

Make sure you purchase and use 'soilless mix' for starting seeds, not a potting mix.
Soilless mix is light so that the seeds can grow.
Most seeds need a warm environment to germinate.
Tall and leggy seedlings are generally due to low light, a grow light is the best way to grow good seedlings indoors.
Provide some air flow when the seedlings have emerged. This helps keep fungus away and grows strong roots and plants.
I even brush my hands gently across the seedlings several times a day.
Make sure the moisture is maintained evenly on seedlings. If they are soggy or allowed to dry out--this is certain issues or death of the seedlings.
I would encourage you to try again, but maybe keep it simple for the first time.

Zinnia and Sunflower Seeds can be easily directly sown into the garden, no need to start indoors.

Pepper seeds can be tough, even for a seasoned gardener.

Tomato seeds are a great one to start with!
Here are some links to help you get started.

https://www.gardeningknowhow.com/garden-how-to/seeds/soilless-seed-mix-info.htm

https://www.gardeningknowhow.com/edible/vegetables/vgen/starting-indoors-direct-sowing.htm
https://www.gardeningknowhow.com/garden-how-to/seeds/when-to-start-seeds.htm
https://www.gardeningknowhow.com/garden-how-to/seeds/preventing-white-fluffy-fungus-on-seed-starting-soil.htm
https://www.gardeningknowhow.com/garden-how-to/seeds/what-is-damping-off.htm
https://www.gardeningknowhow.com/edible/vegetables/tomato/growing-tomatoes-from-seed.htm

I am going to need a little more information. Are these plants in the ground outside (if so - were they just recently planted?) or are they seedlings currently being grown inside? If they are outside, what has the weather been like? Really rainy? Really cold? - what have the lows been?

How often have you been watering them? What kind of light/sun are they receiving?

Some yellowing on the bottom leaves is generally not an issue.

Here are some links to help you.

https://www.gardeningknowhow.com/edible/vegetables/tomato/yellow-tomato-leaves.htm

https://www.gardeningknowhow.com/edible/vegetables/tomato/growing-tomatoes-guide.htm

No you should not use the grass clippings from the lawn that were treated with a weed and feed treatment.
The herbicide may or can carry over the garden soil and plants.
This also goes for compost, do not compost treated lawn clippings.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-long-does-weed-killer-last-in-the-soil.htm

Yellowing leaves, as long as they're only around the bottom of the plant, are usually nothing to worry about when the plant is more mature. This early in the season though, depending on where you're located, there's a good chance it's being caused by the wet weather and lack of sunshine. Those lower leaves simply might not be getting enough sun. This article may help you identify whether that's the case or whether there's a bacterial or pest issue, and treat it accordingly: https://www.gardeningknowhow.com/edible/vegetables/tomato/yellow-tomato-leaves.htm

The straw mulch is a great idea, just make sure it's the right type. This article may help you: https://www.gardeningknowhow.com/edible/vegetables/vgen/straw-mulch-for-vegetables.htm