As Salam Aleikhum brother I am now a muslim I have taken my Sahada as witness .Can I keep my old name as a muslim if I chose to keep my old name for example my name is Shawn.I want some clarification on this.
Basically yes, unless it's an anti-Islam name:
As for the question raised at the end of your letter regarding changing her name, the answer is: if the name of the person has a forbidden meaning, like the name of an idol or one of their deities or a name that means worshipping other than Allah, such as Abdul-Maseeh (worship per of Jesus the messiah) or Abdul-Hussain (worshipper of Hussain) then it is mandatory to change it. However, if its meaning is permissable and not objectionable, e.g. if it means one of the colors such as red or green or a vocation like carpenter or baker, then it is OK and it is not obligatory to change it. However, if changing the name in official documents would cause the person much distress or harm, then it suffices to change it amongst his family and acquaintances.
Moreover, converts nowadays are often encouraged to keep their name. Converting to Islam can be an overwhelming experience, and it may alienate one's family. Thus, keeping one's name is a way of not changing too rapidly, and maintaining ties to one's family.
His family ties might already be strained by his conversion. Many Christian parents see their children adopting another faith as leaving the family. Even though this is not true, him changing his surname will add insult to injury.
I asked a similar question here: How should a convert go about choosing a Muslim-appropriate name? I use "Jannah" as a nickname.