im land owner and have couple who are in default of rent to own agreement, what type of letter notice should i sent first

You should follow the terms of your installment agreement as they pertain to giving notice of default.  The agreement might specify that it be in writing and the manner that the notice is delivered.  You should follow those terms carefully.  If the agreement doesn't speak to how notice is to be given, then I would suggest that you do so in writing.  The notice should state that they are in default. Provide details of the default:  when payments were missed, how much is now due with any penalty or interest, fees, etc.  What amount they must pay and by what date to cure the default and what you intend to do if they fail to cure the default.  You should send that to them via a method that you have proof that you mailed it.  check with the post office.  Please note that installment agreements are governed by PA law.  They must have certain terms in order to be enforceable.  You might want to have your agreement reviewed by an attorney to tell you if it complies with state law.  If it does not, the attorney can help you anticipate potential problems if you need to try and enforce the agreement.