Which starter should i pick!?

The first Gym you will visit is the Pewter City Gym. Brock uses mostly Rock types, so in this situation Squirtle would be the easiest choice. (If of course he is at least lvl 8 and has a water attack.) Blastoise will also be useful in the fight against Blaine, since he uses mostly Fire types.

Other gyms, such as in Lt. Surge's Gym, Squirtle will make battling more difficult, however you will have more Pokemon available to catch at that point. Also in Erika's Gym you will be glad to have a fire type around, such as Charizard, to deal with her Grass types.

Looking at their final evolution forms I can't really choose for you, as both Pokemon have their strengths and weaknesses, I can give you a short summary and a few suggestions however.

Charizard- This Pokemon's Special Attack and Special Defense are higher than his Physical Attack and Defense. Therefore it is best to stick with Fire based moves and have him fight against Pokemon that use Special attacks. He is weak against Rock, Water, and Ice. So take this into consideration. Charizard is resistant to more types than Blastoise however, and he is completely immune to Ground attacks.

Blastoise: Blastoise is similar to Charizard in the fact that his Special Attack and Defense are higher than Physical, so the same general idea applies, replacing "Fire" with "Water" of course. Blastoise is weak against Grass, and Electric and is not immune to any type.

All of the starter Pokemon as very similar in the fact that they are all good. You should decide which one you think looks better, and which type suits you better, in all honesty you can't pick wrong. That being said, I went with Venisaur, but Chairzard was a close second for me simply because it looked like a Dragon.

Hope I helped a little.

Charmander (he evolves into Charizard later on) just on preference when I used to play this game.