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I can't see any signature image, but assuming you mean Sudowoodo (the tree between Goldenrod, Violet and Ecruteak):
Beat Whitney (3rd gym Leader)
Talk to the girl near Sudowoodo
Return to Goldenrod
Enter the house in the northeastern end of the city
First talk to the girl you talked to near Sudowoodo, then to the other one, who will give you a Squirtlebottle which you can use to awake Sudowoodo. If you want to catch it, save in front of it, it can be pretty hard at times.