Thank you for your question about when to start indoor planting in zone 6B. I assume that you are asking about starting seeds indoors that will be ready to plant out when the last average frost date has occurred.

You can look up the last average frost date on this site:
https://garden.org/apps/frost-dates/

Different types of crops (tomatoes, peppers, cosmos, zinnias) require different amounts of time to grow. Some seeds germinate and grow very slowly. Some are quicker. The seed packet will tell you how many days before planting out the seeds should be started inside. For instance, most tomatoes should be started 6 to 8 weeks before the last frost date.

Count back the weeks before the last frost date. If yours is April 15th, then 6 weeks before that date would be March 4th. You would start your tomatoes on that date.

Learn to read the seed packets, consult your last frost date, and count back on your calendar to determine when to start your seeds.

Here's a good document on indoor seed starting:
https://u.osu.edu/knoxcountyag/2018/03/21/a-beginners-guide-to-starting-seeds-indoors/

Here's another good one:
https://extension.psu.edu/seed-starting-demystified

Good luck and have fun with your seed starting.