I am a 49 year old Step mother who wants to dress well for my step daughters high school graduation what do I do?

I'd first ask my step-daughter what she thinks.

I would wear a skirt with a jacket or cardigan, a daytime dress, or nice outfit with dress pants that would be appropriate for church. Don't overdo - keep it tasteful and simple, at least knee length for a dress or skirt. No hat - it annoys people who are trying to see the event. Avoid evening looks like sparkly things, anything low cut or too short - not the time to dress too sexy.