I am registering domain with mydomain. I am confused right now I am bookign with my contact details, can I change it later on with client details?
I'm assuming you are talking about changing whois information. I'm not familiar with mydomain (I use NameCheap). But with any registrar this should be editable. What you could do is have your client set up an account and then upon completion of payment etc. forward the name into his account. At Namecheap you can change the Whois info automatically to the new owner, but at the very least the new owner will be able to change the contact info. Skinny