Hi Nagesh,
After giving all the the necessary information like amount tax code, etc, you can click on Net proposal button ( in this example I have taken FB60). then system calculates tax net amount. Alternatively, you can use doc types like KN, DN where system post tax on net amount.
Please give appropriate subject line so that you can get good response, also search the forum before posting, the same subject might have already discussed in the forum.
thanks,
Venkat