Pokemon-go – How to do with pokemon or candies that I cannot use

This is completely game dependent, and can be mostly correlated to the power of Pokemon you obtain after evolving your Pokemon.

Normal ones like Bulbasaur and Squirtle needs 25 evolution to evolve into Ivysaur and Wartortle, which are middle evolutions, which then need 100 candies to evolve into Venusaur and Blastoise. As there's a great difference between the power and moves of a Wartortle and a Blastoise, Niantic simply decided to set 100 candies for the final level of evolution.

Like Samthere said, you can delay indefinitely, however id recommend keeping one Pokemon high to take gyms with. Don't power it up more than you have to but keep it high enough to get at least a few gyms. The extra star dust and coins you get should be worth the candies and stardust you spend to keep just one Pokemon high. I usually get rid of anything with a CP meter under 90%. It's probably good to set a goal like level 25, otherwise you probably will delay it indefinitely. Lastly, go ahead and evolve any Ratatas and Spearows (or other really common ones in your area) you can for the experience. Their evolved forms max out pretty low so they aren't really viable later, otherwise leveling will get really slow unless you are hatching eggs left and right.