Well you see your going to have to go to bills cottage his grandfather will be there bills cottage is in goldenrod city its in the yellow house right across the street from the radio tower when you talk to his grandfather he will give you a fire stone so it can evolve I did and my growlithe evolved into ARCANINE I loved my arcanine and if you just enter goldenrod city talk to bill he will give you and eevee he wont give you it if your pokemon party is full if you get the eevee get a water stone and then your eevee will evolve into vaperon and then if you have a thunder stone you can let your vaperon evolve into jolteon and then if you have a second fire stone (this may not work but try) you can evolve jolteon into a fire type pokemon I'm not sure what its called but anyway just try it I reapeat (this may not work)
Actually, you give it a Fire Stone. But I would suggest leveling it up to 30 beforehand so you can get some attacks that Arcanine doesn't have if you evolve it at Lvl. 1.
It doesnt evolve at level but with a fire stone
Growlithe does not evolve by level you need to get a fire stone and give it to him/her and for all your pokemon evolution questions go onto here www.pokesplash.net
You level it by giving it a Fire Stone