I want use Google Assistant from my phone to send HTTP POST command to my server. I have a simple webnms app running over it, this server support REST API and now I want to use Google Assistant to shoot GET or POST command to that server and return my output.
Is it something possible? I am not full time developer.
Yes, it is possible.
Your server will need to implement the Actions on Google API. This is a REST API which will accept JSON containing what the user is intending to do and specific information about what they have said. Your server will need to send back JSON indicating the reply, along with additional information about how to continue the conversation.
You will likely also want to use a tool such as Dialogflow to handle building the conversational script and converting a user's phrases into something that makes sense to you. You'll also need to use the Actions on Google console to manage your Action and provide additional details about how users contact your Action. All of this is explained in the Actions on Google documentation.
Simple Actions are fairly easy to develop, and can certainly be done by a developer as a hobby. Good Actions, however, take a lot more thought and planning. Google offers you to the tools - it is up to you to best take advantage of them.