I plan on marrying my boyfiend in Colombia but I was told I need to write a leter from and have it notarized and stamped by the Secretary of State here in Michigan but I want to know what should all be in the letter? I have an idea but want to be good.

Who told you to do this?

The Michigan Secretary of State has no idea whether you've ever been married. You can write a letter to affirm that you are not now married, and never have been. Then sign and date it in front of a notary public who will apply the Michigan stamp (which simply certifies that you are the one who signed the letter, not that what the letter says is true) . Any licensed notary public will do.

I assume you are US citizen. To get married in Colombia, read here:
Marriage in Colombia | Embassy of the United States