Well there's quite a few that you could try.
First off all you have poppies, Honesty, forget me nots and foxgloves. There are many more but these are simple plants.
The foxglove and honesty tend to flower in the second year .
Poppies are easy to collect. Wait for the head to develop and brown off with the holes showing and you simply cut head and shake in a tub to collect seeds. Sprinkle next spring or late spring early summer.
Foxgloves are just a case of waiting for the seeds to show then cutting stem and shaking over a prepared border.
Forget me nots are aggressive seeders. Wait for the flower to finish and produce seed heads then remove mother plant and shake. These tend to take over.
Honesty is a little different where you wait for the seed heads to turn silver and I cut the main plant stem and hang them to dry. Once done rub the seed heads to gather seeds and simply scatter them on a prepared bed.
Here's a border showing a simple layout that gives a early colour using tulips, daffs and forget me nots. There are lilies and bearded irises in there to for later colour.
and another before this where I prepared the bed
Half the battle is ground conditions, weather and then the quality of the seeds.