I recently landed my first job but my client is not responsive and I need some information to start the job so I sent him a message and I am waiting his response but I just notice that he hired two people(including me) for the same job so I am thinking that when I am waiting him to respond the other freelancer may be already working on a job so I am little worried.Should I just start the job based on what I know and wait for his response for further details?
I wouldn't start. The client might be distributing work between you and the other freelancer, and not be happy if you do something that he doesn't need.
Basic rule: never accept a contract if you don't have everything to start working.
Congratulations on landing your first job on Upwork! Martina is right, clients can hire multiple freelancers for one job post to distribute the workload to the team.
I would like to clarify that it's best to agree on all deliverables and deadlines before the contract starts. If you are working with a client and they become unresponsive for a long period of time, it may be best to stop working on the project and wait for contact from the client. You can find more information in this help article.
Let us know if you have further questions.