If you're not having much time right now, I recommend you to skip the D.C.I
I doesn't mean that D.C.I is bad(I'm totally in love with this series, I can remember all heroine name and their backstory both D.C.I and II) but I'm afraid that D.C.I alone without D.C.II won't be great as some legendary game such like F/SN,Clanned and might ruin your expectation
if you're not looking forward to D.C.II and having tight time schedule, you better skip it for now but if you still want to try it out I recommend 3 heroine that you should finish
1.Nemu(Cannon of the game, her route is only inferior to Miharu's but she's the True heroine so you should try her out)
2.Sakura(Key of the game, not the best girl or the best route but she plays a very important role in D.C. series. SHE'S THE BEST GIRL IN D.C.II and the reason I'm really looking forward to D.C.III.)
3.Miharu(Best girl, her route is full of dramas and your tear)
after finishing the D.C. you better grab D.C.II and finish the D.C.II Sakura route ASAP, I can guarantee that it'll worth your time
Thanks for reading and sorry for my bad grammar.