Level it up to level 36. Like all other starter pokemon...
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If it is Rival's Grovyle, evolve your starter a lot.
treecko evolve l16 to grovyle then to l36 to sceptile
at level 16 it will evolve to a grovyle and after that it is 36 to a sceptile.
Level 36 he evolves into Sceptile.
It is not found in the wild, you need to either trade from Ruby/Sapphire/or an other emerald, or choose Treecko as your starter pokemon, and evolve it twice, level 16 for a grovyle and level 36 for the sceptile.