I'm just getting into BPCR'S and my first rifle is a original Sharps 1863 carbine converted to 50-70 and I was wondering what is the best starter load to begin with on these old rifles?

Well, I'll throw in a little loosely configured advice, ...based on what I've found out about "conversions" along the way.

A.) you really need to know the bore / groove sizing,...best obtained by "slugging". Some of the early relines run WAY over the conventional .510, and outwards towards .516.
B.) given the THICKNESS of modern replica brass neck's, this can and will present a problem.
C.) Croft Barker's book about the .50-70 should be considered, "must have."
link: http://www.bpcr.net/site_references/reference_books_relevant.htm
D.) Fireform some DEEPLY annealed brass, and keep it WAY long, through one or two fireformings,....THIS will give you "trim length".
E.) Measuring your fireformed will give you the max bullet dia you can load, ...which may well be smaller than your groove dia.
F.) If that's the case, discard any hopes of shooting a flat base, and start looking for a good conical / hollow base that you can load at case mouth Diameter.
G.) pack the hollow base with bullet lube and seat it against a good THICK veggie fiber wad
H.) 60 grains of just about any good BP is a place to start, ....and you may well find that loading any hotter can induce skirt /base ring separations.

Fun to shoot, and surprisingly accurate, some of em'.

Have a qualified smith check the firing pin and it's general health before going to work,....

GTC

Link: http://www.midwayusa.com/product/729536/50-70-shooters-handbook-book-by-croft-barker

Thanks for the help!!