While battling it, you use a Pokéball on it. If you are successful, you have caught the Pokémon.
Well first of all, Bellsprouts are not available in the following games:
- Fire Red
To get Bellsprouts you need to trade with Leaf Green. The alternative to them in Fire Red is Oddish. To be able to get Oddish in Leaf Green you need to trade from Fire Red. However Oddish are also quite common in Ruby, Sapphire and Emerald.
So basically Bellsprouts are quite rare. They are only available in Leaf Green.
wrong you can easily find them in gold and silver version