"How do I convert over 15,000 American Airlines points to Marriott Rewards points?"
Do you mean convert AA "miles" to MR points? Not sure that AA offers that option. However, AA does offer the option to redeem AA miles for Marriott certificates but the redemption starts at 38,500 miles. There is also a transaction fee. For convenience, the details are posted below.
Marriott Group - redeem for Award Certificates
You may redeem your AAdvantage miles for specific Marriott Hotel awards as outlined below.
Marriott Award Descriptions