I wouldn't flat out ask people to mentor you, just as you wouldn't ask someone explicitly to become 'close friends' right away - great relationships take time to be developed.

Instead, how about simply asking a more experienced entrepreneur or investor for their advise on your startup. Depending on where you live, you could approach them at local startup events (where are you based?) - offer to buy them lunch or coffee in exchange for their advise. Who knows, ideally one of those might become your startup's first investor - as the not-too-old adage goes, ask for advise to get funding, ask for funding to get advise.

Most people will be flattered if you just genuinely ask them for their opinion and help - the rest has to be developed over time.