[SalesForce] Alert message on vf page

You can do like below. I have made a inputText field required through class="requiredInput" and class="requiredBlock" attribute. You can also do something like that. These classes are salesforce native classes so you don't need to create them. Keep on thing in mind that these classes should be under <apex:outputPanel > tag unless they would not work

        <apex:pageBlockSectionItem >
            <apex:outputLabel value="Name on Card" />
            <apex:outputPanel >
                <div class="requiredInput">
                    <div class="requiredBlock"></div>
                    <apex:inputText value="{!name}" label="Name on Card"                     
                    required="true"/>
                </div>
            </apex:outputPanel>
        </apex:pageBlockSectionItem>

You can do all that check in Javascript and then call your apex method (if you have to).

Let's assume this is your picklist:

<apex:selectList id="myPicklist" size="1" value="{!selectedValue}" onchange="myPicklistChanged();">
    <apex:selectOptions value="{!myPicklistOptions}" />
</apex:selectList>

And you have your action function (only if you need to call your apex method)

<apex:actionfunction name="reset" action="{!check}" rerender="pageBlock" />

Finally your javascript function that is called from the onchange event on your picklist:

<script>
    function myPicklistChanged()
    {
        var myPicklistElement = document.getElementById('{!$Component.myPicklist}');
        var myPicklistValue = myPicklistElement.options[myPicklistElement.selectedIndex].value;

        if (myPicklistValue == 'value 1')
        {
            alert('value 1');
            // if you don't want to call your apex method
            // return;
        }
        else if (myPicklistValue == 'value 2')
        {
            ...
        }

        // if you still want to call your apex method after this check
        // check();
    }
</script>