how to get value for search in joomla module

The basics of what you need are in your filter .xml file there is a name= field and that is where the text you want to filter your list will be stored. In the below example it is 'search.

<?xml version="1.0" encoding="utf-8"?>
<form>
    <fields name="filter">
        <field name="search" type="text" hint="COM_INVENTORY_SEARCH_FILTER_SUBMIT"
               label="COM_USERS_FILTER_SEARCH_DESC"
               class="js-stools-search-string" 
               />
    </fields>
</form>

Because haven't answered my earlier question about if you are trying to filter the #__user table I can't be more specific other than to say normally your data base call would or should be be in a model and not a helper. This would extend JModelList that has a few auto-magical things happening in the background to help to wen dealing with data and tables.

Either way in your function to query the table you need to have something like this line to retrieve the value from the search field from the User State object where it should have been stored from your form. // Filter by search $search = $this->getState('filter.search');

And then you need to build add the Where clause to the query with something like this

if (!empty($search))
{
    if (stripos($search, 'id:') === 0)
    {
        $query->where('a.id = ' . (int)substr($search, 3));
    }
    else
    {
        $search = $db->quote('%' . $db->escape($search, true) . '%');
        $query->where([
            'a.ItemCode LIKE ' . $search,
            'a.GenericCode LIKE ' . $search,
            'a.Description LIKE ' . $search,
            'a.SupplierName LIKE ' . $search,
        ], 'OR');
    }
}

There is not enough of your code or an explanation to tell if your Form is correctly passing your value from the Filter/search box to the userState object, but this might be enough information above to help you progress.