I am assigned to prepare a cash flow statement of a new contracing company.What are the thing I have to take in to consideration to prepare cash flow.awaiting for your answer.

They have provided me profit and loss account(starting with notional profit,not included sales or contract cost)

And balance sheet.

This is no easy way to answer how to do a cash flow. It's not really something you can learn everything about and therefore do properly in a couple of days. Most students who are new to them think they are difficult, even though relatively speaking, the ones they do are easy. Ones for real life can get weird. If this is a real life job, I don't understand why they're having you do something like a cash flow if you don't already know how.

Just start doing some reading to get the concept. You're welcome to come ask specific questions about stuff, but you need to get started on something somewhere:
Cash Flow Statement | AccountingCoach.com (http://www.accountingcoach.com/online-accounting-course/06Xpg01.html)
Principles of Accounting Chapter 16 (http://www.principlesofaccounting.com/chapter%2016.htm)

Morgaine is right there is no easy way to do a cash flow.

What I look for beside the basic text book adjustments (e.g. depreciation) is anything on the balance sheet and income statement that might be a non-cash item such as amortization of deferred finance costs or items in Other Comprehensive Income.

Also, you'll need to get the amount in PP&E that actually was not paid as cash but is still in AP or accrued expenses. It is not just the change in PP&E if it was not really paid for.

And I just mentioned a few of categories. You really need to know a lot about the company to do a "real life" cash flow. It takes understanding each line item in the BS and IS and many discussions with other areas of the company. If this is not a school project, I would recommend someone training you or at least providing you with contacts within the organization.