A). For WSDL implementation:

1. Download WSDL by going to Setup----Develop--->API

2. Consume the WSDL to your  application (It will generate stub classes there)

3. make a Login call to fetch session ID, store session ID to make further calls.

For detail information of calls, check Core calls section in WEB service API DOC of salesforce

B). For REST API implementation:

1. Configure "Remote Access" within salesforce, Setup--->Develop------>Remote access.

          a). Give your application a name

          b). Provide a call back URL, If you are developing window based application then you can give any string as a URL. However, if you are using webbased application than you have to provide the exact URL of your site or application, i.e hhtps://test@test.com.

Leave all other field empty and hit save.

After clicking save Salesforce will automatically generate Consumer Key and Consumer secret.

use Consumer Key , Consumer secret and Call back URL and make HTTPS post call to salesforce,  salesforce will return a token (called Oauth token gernally). save this toke and use it to make further calls in salesforce.

Note : Oauth token  is comibination of session ID , UserID , OrgID and other vital information.

Hope this will help you to gave startup, if you need further help please contact me at : 1508.mishra@gmail.com

Regards,

Pankaj Mishra