You could start here:

http://java.sun.com/products/jndi/tutorial/index.html

Well it won't quite be that easy. I'd suggest before trying to interface
with an LDAP server programmatically you read about the protocol itself
and how directory servers operate. Ideally you should attempt to
interface with a directory server using the native tools for a
particular directory server.

Short of that you need to do the following:
1. Determine the type of LDAP directory server you will be interface
with: Active Directory, OpenLDAP, Novell eDirectory, Oracle Internet
Directory, Sun One, etc. (there are a couple others but they aren't
popular).

2. As long as you only need to do searches on the objects in the tree
and not the schema then you can actually use JNDI (from the JDK) for at
least Active Directory, OpenLDAP, and Sun One. My app uses JNDI for
those 3 for read-only access. If you need to make changes then JNDI will
only work for Active Directory and OpenLDAP.  To make changes to Sun One
you have to use the netscape Java API. There is a JNDI provider that may
work (I haven't tried it yet though).

3. Within your actual code you need to setup a Hashtable that contains
values you will use to connect to the server:
    ldapEnv.put(Context.SECURITY_AUTHENTICATION,"simple");           
    ldapEnv.put(Context.SECURITY_PRINCIPAL,username);
    ldapEnv.put(Context.SECURITY_CREDENTIALS,password);
    ldapEnv.put(Context.PROVIDER_URL, "ldap://" +
                    host + ":" + port);

4. Setup an InitialLdapContext using the Hashtable:
    ctx = new InitialLdapContext(ldapEnv,null);

5. Then you need to setup your search parameters:
attribs is a comma-delimited list of attributes you want returned in
each object of the search results (e.g. givenName, sn)

public Vector<SearchResult> search(String base,
        int resultLimit,
        int searchTimeLimit,
        int scope, String filter,
        String attribs) throws Exception {
    NamingEnumeration results = null;
    SortControl reqControl = null;

    String[] attrs = null;
    attrs = attribs.split(",");
       
    SearchControls searchControls = new SearchControls();
    searchControls.setReturningAttributes(attrs);
    searchControls.setSearchScope(scope);
    searchControls.setTimeLimit(searchTimeLimit);
    searchControls.setCountLimit(resultLimit);
    try {
        reqControl = new SortControl("cn",true);
    } catch (IOException io) {}
    ctx.setRequestControls(new Control[] {reqControl} );
    results = ctx.search(base, filter, searchControls);
    Vector<SearchResult> sortedResults = new Vector<SearchResult>();

    while (results != null && results.hasMoreElements() ) {
        sortedResults.addElement((SearchResult)results.next());
    }
    results.close();
    return sortedResults;
}

Parsing those results is a whole other matter that you should be able to
do yourself.  Look at the javax.naming.directory.SearchResult class.

Post again with more specific issues and I can help you more.