Does joker.com host your website? Does mindspring host your website or is it one of those free personal pages?
If mindspring hosts your pages [ie. you paid for hosting], they should have given you DNS numbers that you give to your joker.com registration. Thru the magik of MX & SOA etc etc, a person reaching
http://www.yoururl.com will be directed to your space on mindspring.
If its just a free personal page then you must
OPTION 1: get someplace like joker.com to host your webpage and then have it forwarded to your mindspring site [per mairving's advice]
OPTION 2: ask your registrar, joker.com, they might have something called "web forwarding" that will redirect your URL to that page (at a nominal extra charge). It should have something like a URL gripper that will not reflect your address on mindspring, so you can potentially change from mindspring free pages to geocities, and all u would need to do is change the information on joker.com
HTH