First you need stool evaluation and possibly colonoscopy with biopsies

If no infectious /inflammatory cause found then would trial a binding agent like colestipol with or without a small dose of psyllium daily

This depends on many factors, especially how much colon was removed and what part of it.
In general, Imodium or loperamide can help. Several other medications are available but their use is based on the specific cause of diarrhea in this setting.