Hi Katerina and welcome to the forums. Could you let us know a little more about your situation? For example, are you applying to a University here? Do you have a degree or a diploma?
Your best start for researching this is at the Ministry of Manpower site. You'll find that here
http://www.mom.gov.sg and look through the site.
I personally am unaware of agents that will assist - however I am in a totally different situation. Maybe some of our other regulars will be able to assist you better.
HTH, Mr. P