How to become IFS officer (Indian Foreign services)

and my doubt about job is about the posting place(India or abroad), for what period, in what(embassy or consulate or mea) and permanent missions/deputations to un and other such bodies ?

Answer
you don't have to concentrate on any specific thing to get IFS.
Once you clear the prelims, you'll be given the mains application form by UPSC.
In mains application form, you've to give service preference (eg. IAS>IPS>IFS>IRS whatever you like)
After you get a decent score in mains, you'll be called for interview, finally your mains interview marks are combined, and a merit list is prepared.
You'll be allotted to a service depending on 4 factors combined,
1. Your preference in the mains form
2. Your merit number (+reservation if applicable)
3. the vacancies in the given service.
4. Medical checkup
that's all.

Now, some people believe (and spread rumors) that to become an IFS officer (Indian Foreign services)
1. You need to select a specific optional subject (like political science & international relations),
2. You need to be Post-graduate from a specific stream or a renowned college (DU, Stephen's, Xavier etc)
3. You need to write the exam in English, you need to give interview in English etc.etc.
None of above is true. All baseless rumors and lies.

Let me quote Hindustan Times article

Last year (2008), a group of candidates with Hindi as their first language made the cut at the civil services exams, and were selected for the Indian Foreign Service (IFS).English is not their strong point and they answered their papers in Hindi, one of the language options available to a candidate. So, for the first time, English is being taught to these six or so young IFS probationers at the Foreign Service Institute (FSI), Delhi.

In short, Exam remains one and same for everyone and every service offered by UPSC's civil service exam. Your selection basically depends on your service preference+ merit number.
Same way to become an IPS officer, one doesn't need to be extremely physically fit athlete / State or national Sports champion / Post-graduate in forensic science or criminology.

Your second question :Job profile of IFS
Copy pasting from another article on this blog,

After the training at Missouri, the I.F.S. probationers are attached to the Ministry of External Affairs and have to become conversant in a major foreign language. During the second year they are appointed as Third Secretaries in and Indian Embassy/High Commission where that language is spoken. They spend another two years in the same Embassy as Second Secretaries. After two postings abroad, the I.F.S. Officers are posted in India in the Ministry of External Affairs (MEA) (Eg. Passport office or Indian Council for Cultural Relations or elsewhere in the Government of India). In the MEA, you will look after India's political, economic and commercial work. In the Senior Scale and the Junior Administrative Grade the I.F.S. Officers are appointed as First Secretaries. In the Selection Grade I.F.S. Officers serve as Counselors. In very small countries the Indian Ambassador would be in that grade.
In the suppertime scale, many I.F.S. Officers become Ambassadors of medium sized countries while others are appointed as Ministers in the large embassies like Washington and Moscow. In the Additional Secretaries Grade, I.F.S. Officers are made ambassadors in relatively big embassies of Deputy High Commissioner in London. The biggest Indian Embassies / High Commissions like Moscow, Washington and London are headed by Ambassadors of the rank of Secretary. At this level, some Ambassadors are non I.F.S. Officers like politicians, retired or serving I.A.S. Officers or retired Defense Chief. There are four to five Secretary level officers in the MEA � the senior most being the Foreign Secretary.
The content of an I.F.S. Officer's work has improved immensely. The is considerable economic work to be done to promote trade and commerce. The career development in the service is extremely professional.