visa possibility in singapore

Dear Sir/Madam,
I hope this finds you well, my intention is to visit China in the coming months. As I am already on my travels I would like some advice on where a good place to obtain the via would be. In July I will visit Singapore and hope its possible for me to obtain it there. Also could you please give me information on a single visa and a double visa and do i require proof that i have no criminal convictions.
Thanks for your time,
Yours Sincerely,
Conor Rotherham