New to being a landlord. My wife and I just purchased three duplexes in the state of Missouri for investments. All units were occupied when purchased with leases.
Upon taking possession, one renter, who had only been there one month under old management, decided that she could not live without her dog. It specifically states 'No Pets' in lease. What are my first steps? What type of documentation should I give these tenants to make life easier down the road? And were online can I get a copy? Someone mentioned that I need to serve them a "Notice to Perform", is this correct? Thx for helping.

Well a notice to perform usually comes after you address the problem by talking to the tenant, writing letters, etc. Try getting pet agreement This document not only lists the pet’s information (Name, weight, age, etc.) but also clarifies the rules and regulations for the pet. It is important to explain to the Tenant that each pet must be accounted for to prevent unauthorized pet(s). Allowing pets may make your property more appealing and easier to rent. You may want to consider collecting an additional security deposit or increase the rent when allowing a pet; but it is important to check if there are limits or restrictions in your state. The advantage will be that any damage to the rental unit from a pet is solely the responsibility of the tenant and must be immediately repaired, cleaned and/or replaced at the tenant’s expense. The agreement form you can get on ezlandlordforms.com

Keep in mind that if you are evicting for rent-it's far more black & white as far as the courts are concerned.

If you are evicting for any other reason than rent-it's a lot harder and you have to documentation that you tried to correct the problem outside of court.

This could a tough one because this tenant can go get a note from her doctor saying she needs the animal as a "therapy dog.." and it could get dragged out in courts for months.

Unfortunately-at this time the "therapy" animals are sort of a new thing and there isn't any legislation covering them yet that I've heard of. HOWEVER-if it's an "assistance animal" you can't turn the person down for rental or evict them.

Try reasoning with her verbally first and follow-up with a letter.

Good luck. It might wind up being easier to just collect a pet deposit and go from there.