Congratulations! It's a great synth. My best tip would be to experiment as much as possible, get to know it as best you can. Dive into all menus and get a good overview of where all the functions are, so you have the "map" in your head and so your fingers know what buttons to push to get anywhere fast. Use presets / other patches as starting points and try to make different sound from that. You will find that using the numerical keypad is great for entering values fast, I prefer it over the joystick but you have to get used to it. After a while you can predict what a specific value will sound like.
Keep it simple at first and disable all but 1 "partial". Try to make that partial as full and complete sounding as possible, then enable the second partial, program it so that it complements the first partial, and you have a "tone". Having "tones" that sound good on their own will help you later when adding two "tones" into a patch, because then you already have a strong foundation and your patches will sound great!
"Partial" is the smallest building block of the sound. You should treat them as a individual synths. If you manage to make a really nice sounding partial, that will leave room for so much more later. Instead of programming 4 very weak partials and relying on adding them together in a patch to make it sound nice. You usually end up with mediocre results. Unfortunely, a lot of commercial D-50 patchbanks were made that way.
Don't bother reading the manual from cover to cover, instead use it as reference.
Getting to know your D-50 is a journey, and it will take time. But it is rewarding.
Bobby Blues provides a nice quick overview:
http://bobbyblues.recup.ch/roland_d-50/ ... ption.html