I know how bitcoin transactions are built 4 Bytes version & 1 byte number of inputs ...etc similar to this how is stellar transaction built and signed ? Please help me find that information ? where should I start looking ?
Don't know in deep detail but... afaik a stellar transaction is a bunch of XDR (External Data Representation Standard) encoded data. I'm not aware of an official definition document but you can find definition files in the stellar base libraries (for example js-stellar-base or ruby-stellar-base). The transaction is Ed25519 signed and tx+signature are combined in a XDR TransactionEnvelope structure. Finally the resulting XDR chunk is usually base64 encoded for transfer.
I think it's pretty complicated to do this by hand on the lowest binary level. If you're looking for a higher level entry, the stellar laboratory is an excellent tool to build a transaction XDR string, sign a XDR string or decode a XDR string.