"Which social media like facebook, twitter.... is the most important for adult SEO?" probably not more important but at least more adult friendly: sex.com
"How much should I spend on adult SEO every month to get good results within 3 to 6 months?" as much as your pockets can pay, good SEO is not cheap, why dont you try to buy traffic?
"Any other tips when I am starting SEO for my website?" yes get as many relevant contextual backlinks as you can.
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