First, you defeat the gym leader Whiteney. Then you get the badge from Whiteney and go to the house at the right hand side of the gym and talk to the lady watering plants.
She will give you a squirtbottle. Cross the National park to the tree with two green colour buds sticking out. Talk to it. It will attack you. be sure to have a lot of pokballs to catch it!
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You can get it by buying it from the Department Store in Goldenrod City or get it from someone that was near the Sudowoodo.
if you know the sudowoodo blocking the way u have to battle it and catch it
The same place you find him in gold, north of Goldenrod City. You have to beat Whitney and then get the Squirtbottle from the house behind the gym. Use it on Sudowoodo and he will attack you.
you don't
Sudowoodo is not the only Pokemon that can learn Rock Smash. There are many Pokemon that can learn it. Most fighting type Pokemon can learn the TM. So no, you do not need Sudowoodo.