First, go see every pokemon you can get in Sinnoh or whatever region you're in.
You don't need to catch them, just see them.
Then, go to the professor who gave you your starter pokemon and he will give you the national dex. :-)
Chat with our AI personalities
Yes, if you evolve a Pokemon from the national dex while you are still using the sinnoh dex then all forms you have accessed previously, whatecver dex you are on, can be found on the dex. Hope this helps!
There are 210 in the Sinnoh Dex, 493 in the National Dex {including EVENT Pokemon}
You get the national dex when you beat Cynthia. And get the poke dex at Rowan's lab.
from proffeser rowen after you get the sinnoh dex filled
you compete the sinnoh dex.