You do not have to worry about rerolling as Pokemon Go does not have a Gachapon system. Do not worry about not having the strong pokemon at the beginning. You can casually start the game without heavy investment.
Spend money on special items.
Pokemon Go does not have a Gachapon system. But it has an In-App-Purchase feature called Pokecoin. You can IAP various items that enhance your catching ability or make the monsters appear more.
You can select not only Bulbasaur, Charmander, or Squirtle, but also Pikachu in Pokemon Go.
How to catch Pikachu at the beginning
Create your Avatar.
Do not pick any Pokemon from 3 starters.
Walk away about 50m (165ft). 3 monsters pop up again.
Walk away again. Repeat step 3-4 three or four times.
Pikachu appears on the screen!
Pick the one that is hard to obtain in your area.
The three starters and Pikachu can be obtained on the ordinary streets. Bulbasaur inhabits in bush or forest. Charmander inhabits in hot area. Squirtle lives nearby water. Check your surrounding area and see which pokemon is likely to appear less. Pick the one that is hard to get for you.
After making your Avatar, pick one Pokemon from Bulbasaur, Charmandar, or Squirtle.