How do you evolve growlthe

You have to get a Fire Stone and use it on him. Depending what Pokemon color you are playing, a Fire Stone will almost be always found at the Mall, which is most often available in the town that the 5th or 6th gym leader is in. It can sometimes be obtained by finding it on the ground before you get to the mall. But you won't find one lying on the ground if your playing fire red or leaf green or any of the earlier games. You just have to buy it. If you get to the mall, I estimate it being sold anywhere around the 3rd-5th floor (along with the other stones, such as lightening stone, water stone, ect..) Just talk to all the people behind the counters until you find the right one. GOODLUCK! =)