Get Some Help
You may not be a good writer, and possibly, you're even a worse public speaker, but don't panic. There's no shame in enlisting friends or family members to help you with your material and delivery. Or, you can opt to go the route of short opening remarks, confessing to not being the best speaker, and then defer to others to fill out the remainder of your toast. Chances are the guests will find it endearing and appreciate your efforts to do something out of your comfort zone.