Masturbating is 100 % safe and there's is no way to get an STD simply by masturbating.
It is impossible to get any type of infection or disease or STD "Sexually Transmitted Disease" from masturbating from the simple act of touching your own genitals.
To get an STD you would have to have sex with someone else who had the STD but if you don't already have the STD then you would not be able to come down with an STD or other sexually transmitted disease through masturbation.
Masturbation is perfectly safe and actually very healthy for you to do and you may even prevent prostate cancer by masturbating daily.