How to digitally sign the message using digital certificate and BaseEncode

Hi All ... I am working on webservices....
I need to digitally sign a message in which AcknowledgementNo using digital Certificate with X.509 certificate private key.
I tried this code but it showing invalid signature Encoding. Can anyone pls us to digitally sign.
Here is the below Java code to sign the AcknowledgementNo,Pls help me.
Thanks in advance.

public class DigiAckNo {
    static class PrivateKeyAndCertChain {
        public PrivateKey mPrivateKey;
    }

    private static KeyStore loadKeyStoreFromPFXFile(String aFileName,
            String aKeyStorePasswd) throws GeneralSecurityException,
            IOException {
        String PKCS12_KEYSTORE_TYPE = "PKCS12";
        KeyStore keyStore = KeyStore.getInstance(PKCS12_KEYSTORE_TYPE);
        FileInputStream keyStoreStream = new FileInputStream(aFileName);
        char[] password = aKeyStorePasswd.toCharArray();
        keyStore.load(keyStoreStream, password);
        return keyStore;
    }

private static PrivateKeyAndCertChain getPrivateKeyAndCertChain(
            KeyStore aKeyStore, String aKeyPassword)
            throws GeneralSecurityException {
        char[] password = aKeyPassword.toCharArray();
        Enumeration<String> aliasesEnum = aKeyStore.aliases();
        if (aliasesEnum.hasMoreElements()) {
            String alias = (String) aliasesEnum.nextElement();
            Certificate[] certificationChain = aKeyStore
                    .getCertificateChain(alias);
            Certificate certificate509 = aKeyStore.getCertificate(alias);

            PrivateKey privateKey = (PrivateKey) aKeyStore.getKey(alias,
                    password);
            PrivateKeyAndCertChain result = new PrivateKeyAndCertChain();
            result.mPrivateKey = privateKey;
            return result;
        } else {
            throw new KeyStoreException("The keystore is empty!");
        }
    }
 
private static byte[] signAckno(PrivateKey aPrivateKey)
            throws GeneralSecurityException, IOException {
        String DIGITAL_SIGNATURE_ALGORITHM_NAME = "SHA1withRSA";
        Signature signatureAlgorithm = Signature
                .getInstance(DIGITAL_SIGNATURE_ALGORITHM_NAME);
        String strAckNo = "41516019";

        byte[] ba = strAckNo.getBytes();

        signatureAlgorithm.initSign(aPrivateKey);
        signatureAlgorithm.update(ba);
        byte[] digitalSignature = signatureAlgorithm.sign();
        // Display plain text and signature
    return digitalSignature;
    }

public static String encodeBase64(byte[] rawData2) {
        return Base64.encode(rawData2);
    }

public static void main(String args[]) throws GeneralSecurityException,
            IOException, CertificateException {
        String File = "d:\\projects\\GANGADHARAN.pfx";
        String pwd = "1235";
        KeyStore keyStore = loadKeyStoreFromPFXFile(File, pwd);
        PrivateKeyAndCertChain PrivateKeyAndCertChain = getPrivateKeyAndCertChain(
                keyStore, pwd);
        PrivateKey aPrivateKey = PrivateKeyAndCertChain.mPrivateKey;
        byte[] bytestr = signDocument(aPrivateKey);
        String strDocumentBase64 = encodeBase64(bytestr);
        System.out.println("strDocumentBase64" + strDocumentBase64);

    }

}