Hi Shakeel,
I am not sure where you are located and what your plans are.
I am assuming you want an official school contact with the ISS.
First you need to apply to ARISS for permission to get an official school contact. During the application there are options for telebridge (using a telephone connection) and direct (using a radio)
For direct (voice only) contacts the recommendation is to have the best station possible. The reason for this is you need to be the strongest station for example in the USA, to try and prevent any accidental interference from other stations transmitting.
Normally for a voice only contact you need 144Mhz 2 metres voice only equipment FM modulation.
Normally the recommendation is to have 2 stations (main and backup). The backup is normally a very low gain omni directional antenna such as a 1/4 wave whip. You normally connect this to your radio with low loss coaxial cable and ideally use a 100W radio.
For the main station what is recommended is azimuth and elevation tracking, such as a Yaesu G-5000. The beam is normally recommended to have circular polarisation at least 6 elements. It is better to have the option to switch from left hand to right hand polarisation during the middle of the contact as the polarisation can change. You also need high power and low loss cable. The transmit and receive frequency should be doppler corrected by computer control.
Normally you would also have an audio amplifier for the room and projectors and recording equipment.
All these are not essential, but very important to be sure of a reliable and long duration contact.
For video conference there is much more work. Firstly the system is not approved for use yet, but might be very soon. Secondly only having one station would limit the video to approximately 6 minutes maximum. For this reason we have many video ground stations in Europe and the plan is to send video to the school over the internet. The school would need good internet. If it is a direct contact the school would need to be in Europe (until USA ground stations are built)
If we assume you are in Europe, and we assume money is not a problem, and we assume you want a direct contact, you could rely on "telebridge video" via the internet for the video feed. If you also want you can also receive the video at the school live and use it for some of the contact, and also use the video over the internet.
To receive the video direct there is lots of requirements. I myself am one of the ground stations for Europe. You need a dish, feed, down-converter, special DVB-S satellite card and lots of time to configure and test a computer to be sure it works. You also need a different rotator for the dish. We normally do not recommend to use the same rotator as the VHF antenna. The requirements are very critical. The dish needs to be accurate to 6 degrees, which the Yaesu G-5500 has trouble with. We are testing other rotators, but also having trouble with these. We hope to recommend a different rotator, when it is tested. You would want a budget of $2000 to $3000 to consider this. If you are based in the USA there is probably no possibility for a direct video contact in the next 1 year.
Beware organising a school contact is a lot of work. You normally need many people and money. You also need a school with a lot of children to be given priority. You also need to educate the children on science and technology. It is not as simple as filling out a form.
The general information for ARISS is here: http://www.arrl.org/hosting-an-ariss-contact and http://www.ariss.org/ More info here: http://www.ariss-eu.org/ Lots of info here: http://www.ariss-eu.org/schoolcontacts.htm There is a link on that page with a form for putting in the information for both main and backup radios.
Information on HAMTV: http://www.ariss-eu.org/columbus.htm
Note the deadline for USA is end of October which is only a few weeks away. You normally need to have the school principal to agree before this date.
For Europe the next submission window extends from February 1 to April 30, 2016.
The better your proposal (better radios/antennas/more children/more education) the better the chance you have of being picked.
If you have any questions please ask.
Dan EI9FHB