how can I run a service for android and ios from my main application?? Help me
There's dozens of tutorials on android services. (HINT: Its not a Xamarin-specific thing so look for how it is done natively)
So I'll try to give you credit for having done some research and made an effort on your own before giving up and needing help.
What have you tried so far and what problems specifically are you running in to that you need help with? Posting your project in a public GitHub repo does wonders for people trying to help you because they can then download your test app and examine it in detail.
Xamarin Inc., as a wholly-owned Microsoft subsidiary acting as a separate legal entity, adheres to the Microsoft Privacy Statement: Privacy & cookies