Rent your bike from your hotel. They will arrange for the bike from a trustworthy shop or person. Usually they will produce an application form that will state for the lease of the bike and a few stipulation and rules.
I rented mine from Anyavee Tubhaek Beach Resort, they arranged a bike for me from an outside shop. The deposit was 2000 Baht and rent was 350 Baht per day. over 6 years ago
I've been in Thailand last december/january. when in Ko Lanta, I rented a car through the hotel desk. The car was delivered and returned at the hotel, but for motorcycles it works the same way. The rental was only paid at the end and to the hotel.
Yes, there's a lot of scammers in Thailand, usually people who come to you in the streets near touristic places offering something you didn't ask for. But just don't be naive and you do fine.
Also, Ko Lanta has a relatively low incidence of scammers compared to places like Bangcoc or Phuket over 5 years ago