I going to start my career as an BA and I would like to know that what can I expect during first month of my job. Thanks in advance

You will be considered as a change agent. That is someone who will come into the organisation to help identify areas for improvements within its business processes and IT systems (that is if you were recruiter with the expectation that you will be involved on the technical side). You will also guide them by recommending and selecting suitable solutions and guiding them through the change process from the old (legacy) system / process to the new systems / process with as little or no risk at all.

You will be involved in a lot of requirement gathering, requirement documentations, analysis and some element of design depending on the nature of the project.

Your role will interface between the business and the IT / development team. Also you will work with all stakeholders across the organisation to achieve your objectives, including a project manager. So it’s very interesting and interactive.  

Just to add, dldelancey is indeed right, sometimes your manager or in fact the organisation may not know why in particular they hired you and one of the reason is because your overall input contributes to the development and implementation of a solution of which the development and implementation is usually handled by a different team except you are asked to take up the responsibilities of a BA and a Project Manager.

I hope this helps.