The first call that you need to really work when using the AdWords API is getAllAdWordsCampaigns. Here's the minimum XML / SOAP that I've found to work. Of course, you need your own AdWordsAPI token and account.
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"> <soap:Header> <email>EMAIL</email> <password>PASS</password> <token>TOKEN</token> <useragent>AGENT</useragent> <clientEmail>CLIENT</clientEmail> </soap:Header> <soap:Body> <getAllAdWordsCampaigns> <dummy>0</dummy> </getAllAdWordsCampaigns> </soap:Body> </soap:Envelope>