Hi Rakesh,
Be confident that you were end user, because you understand most of the business processes in US Scenario. The important points to get started include (if you want to continue on US):
1. Knowledge of all the Infotypes both common and US specific.
2. Positive time scenarios and their implications.
3. Customizations out of standards which were implemented when you were user(requirement and the PCR for the same, or BAdIS).
4. Going through the PCR/Schema F1 Help docs, as you already analyst you have access to an implemented system. As a small step towards excellence start off with two schemas per day, go through each function of the schema and its parameters try to understand them, if stuck some where SCN is always there, post it.
5. Don't try to gobble up all in one day, that will not be of any help. Try to take each step day by day, when you are done with one schema or pcr, you should be able to figure out the purpose of the PCR/Schema, functions/operations used in it, if the same is flashed on the screen.
6. Then comes the Benefits, taxation and BSI part, there is lot of stuff in SCN regarding the same, try to search it in the forum/SAP Market Place/BSI online documentation and if not through then again SCN is always there, post it.
And refer to this link New to SAP HCM? Where to Start? a good post by Vivek to go through for Starters like you.
Hope this helps, and try to post your answer/query point wise so that the same can be cleared systematically.
BR
Upen