After saying the takbeer, you should go down into the position of prostration on your hands first, putting them down before the knees. This is what the Messenger of Allah (may the peace and blessings of Allah be upon him) did. He also forbade imitating the manner in which a camel sits down, which is by kneeling with its forelegs first. It is also mentioned that the camel descends with some force, and therefore it is required that a Muslim descends to the position of prostration in a controlled and gentle manner.

Abu Hurayrah reported that the Prophet (may the peace and blessings of Allah be upon him) said: 

"When any one of you prostrates, let him not go down as the camel does; let him put his hands down (first) before his knees." [Abū Dāwood: 840, an-Nasā'ī: 1091]

Note that this issue is one in which many of the scholars differed, and there are other narrations which seem to indicate that it is a sunnah to put down the knees first; however, after a study of the authenticity of each narration, the narrations which mention the knees first have some weakness, and the narrations of putting the hands first are more authentic, and Allah knows best.