Hi Mahendra,
There are two view points to your query. Since you have already joined a small BPO with 20 odd employees, dont look back and reconsider your decision. Life is full of challenges and dont expect cake-walk all the time. Use this opportunity study, learn everything that is required for setting up a proper HR department. This way, being the first HR guy, your successors will praise you, if you have done a thorough job. Use your spare time to find out from your employers, how to make the HR department robust, what is required, develop a blueprint / plan for the next 5 years. Gradually start implementing the plans like establishing HR policies, procedures, systems, rewards, performance review etc etc.
Second aspect is that you work for a bigger company and be part of the already existing HR team, learn things and move on to another company and implement things you know and you learnt in the earlier organisation.
For the first suggestion that I made, I would suggest that you take me as your HR Consultant who can help you in establishing a proper HR setup, including letter formats, templates, etc etc. You can check my profile. Please remember, nothing comes free in this world, there will always be a price tag attached to the services !
Best wishes