question about schema

Dear Sir:
    I'sorry for bothering you, but will you please do me a favour?  
    I'am taking advantage of Xml to develop a project. When i write the Middle Layer Program, i need to write a xml schema to parse the xml i get from the client program for the correctness of the xml string.then there are some question i confront:
    1. whick schema should i choose? Does the DTD type seem old and less constraint? 
    2. Can i choose .xdr to be my Schema?the project i developing concerned with Sql Server, Vb, and maybe EDI further.
    3. If i choose .xdr, then can i put the Schema into the  xml string? if it can , how?

    That's all , Thank you very much.
    i'm longing for the reply letter.
                                                                                                                Jianfeng Sun
                                                                                                                2001.5.9