Typically, you will define an Apex action function for your Visualforce page that does the callout. Here's a good article with the basics of Apex Web Services: Apex Web Services and Callouts (http://wiki.developerforce.com/page/Apex_Web_Services_and_Callouts).

Here's an example of calling a web service to get postage rates:

    public PageReference makePostageLabel() {
        EnvmgrLabelService.LabelRequest labelRequest = new EnvmgrLabelService.LabelRequest();
       
        labelRequest.RequesterID = 'abcd';
        labelRequest.AccountID = '123456';

        labelRequest.MailClass = mailClass; // Controller properties - bound to fields in VF page
        labelRequest.MailpieceShape = mailpieceShape;
        // etc

        // Get the SOAP service
        EnvmgrLabelService.EwsLabelServiceSoap service = new EnvmgrLabelService.EwsLabelServiceSoap();

        // Call the web service
        EnvmgrLabelService.LabelRequestResponse labelResponse = service.GetPostageLabel(labelRequest);

        // See what happened - status, postageLabel, finalPostage can also be bound to fields in a VF page
        if ( labelResponse.Status == 0 ) {
            status = 'Label created';
            postageLabel = labelResponse.Base64LabelImage;           
        } else {
            status = labelResponse.ErrorMessage;
            postageLabel = blank;
        }
       
        List<String> format = new String[]{'0','number','###,###,##0.00'};
        finalPostage = String.format(labelResponse.FinalPostage.format(), format);

        // Return null to stay on the same page and show the result. Could also send the status etc to
        // another page - set parameters on the PageReference
        return null;
    }