Since, this question is for v3 API access, you need to access- developer.intuit.com/apiexplorer?apiname=V3QBO
Refer to the ouath 1.0 docs to generate the signature as mentioned below:
http://oauth.net/core/1.0a/
http://oauth.net/core/1.0/#signing_process
Twitter example for generating signature-
https://dev.twitter.com/docs/auth/creating-signature
Sample RAW REQUEST AND RESPONSE headers-
Request:
Authorization: OAuth oauth_token="CCCDOPwLGymIrFSHVZ317Qx",oauth_nonce="cddde-464b-458a-a718-2dcd8062616a",oauth_consumer_key="qyprtysdsdgJZUvWDFnPoiBh1ejZn",oauth_signature_method="HMAC-SHA1",oauth_timestamp="1379398521",oauth_version="1.0",oauth_signature="mwVUkoHnh%2BPLqZolso2rzX5aABM%3D"
Response Header:
Cache-Control : max-age=0, no-cache, no-store, must-revalidate, private
Connection : close
Content-Length : 1402
Content-Type : application/xml;charset=UTF-8
Date : Tue, 17 Sep 2013 06:27:48 GMT
Expires : 0
intuit_tid : 0f9b5431-b713-4d55-ac24-c00f4e91848e
Server : Apache/2.2.22 (Unix)
Vary : Accept-Encoding
Via : 1.1 ipp-gateway-.net