press "/" sign and then press "e" to type "ya" when u are using an english keyboard
st is not any character or sound it is either ste or s, it is the combination of half "स" = स् and त
here is the combination u can use to type नमस्ते :: vcmdls
And the last one.. Most people write larka or ladka inplace of lardka (as in लड़का (boy)) you can press " [ ] " keys (without space) to use the sound rd as in ड़.
Let me know if that works.
Thanks