1 If you are a novice you should not be designing a structure of this magnitude on your own. Sub-contract it.
2 British Concrete Association have many good examples of PSC design. I am sure that the American equivalent will also have similar examples.
3 The beam design is just the start.
4 Sub-contract or employ someone with experience.
5 Employ someone with experience.
6 Should you be unsuccessful in 4 and 5 get reading. ICE proceedings, ACI publications, etc.
Good luck
I cannot overstate my agreement with OLDHAND on this one. You need to either be mentored by a more senior engineer or have a bridge engineer consult on the project.
Box girders don't come along every day and there is much that is left beyond the simple design to determine the number of strands necessary to support the dead and live loads.
One such concern is the camber of such a beam and how the slab will be set with that in mind.
Another is at 50 meters you will undoubtably need some type of falsework and will either be responsible for that or will have to review the falsework to make sure it doesn't conflict with your design.
A 50-m span cannot be transported w/out splicing the girder. Therefore, the individual girder pieces will require prestressing but post-tensioning will be required to put the pieces together.
The other option would be to cast on-site.
Or, the other option would be a cast-in-place PT girder.
IMO this should be left to experienced bridge designers.