New to this. Seeking advice

I have being in a relationship with my girlfriend for 2 years and her 9 yr old son has high scale autism. I'm completely new to the whole autism spectrum and feel in the dark how to cope and handle various situations. 

I have a 8 year old girl from a previous relationship, but cannot help but compare the two when dealing with different events such as tantrums or social occasions. I constantly feel paranoid when going public places, meltdowns at home or out and about. 

I would really like some advice how to deal with meltdowns and how to build a bond with the child.

I will be moving in with my partner soon as we are expecting a baby, and this is also a worry for me as I don't know how he will cope with a big change in the home.