Salesforce create custom controller

1. Go to "setup/Develop/Apex Classes" and click "new"
2. Write the following code:

public class CustomController {

    public Account account { get; private set; }

    public NewAndExistingController() {
        Id id = ApexPages.currentPage().getParameters().get('id');
        account = (id == null) ? new Account() :
            [SELECT Name, Phone, Industry FROM Account WHERE Id = :id];
    }

    public PageReference save() {
        try {
            upsert(account);
        } 
        catch(System.DMLException e) {
            ApexPages.addMessages(e);
            return null;
        }
        //  After successful Save, navigate to the default view page
        PageReference r = new ApexPages.StandardController(Account).view();
        return (r);
    }
}

3. Go to "setup/Develop/Visualforce Pages" and click "new"
4. Write the following code:

<apex:page controller="CustomController" tabstyle="Account">
    <apex:form>
        <apex:pageBlock mode="edit">
            <apex:pageMessages/>
            <apex:pageBlockSection>
                <apex:inputField value="{!Account.name}"/>
                <apex:inputField value="{!Account.phone}"/>
                <apex:inputField value="{!Account.industry}"/>
            </apex:pageBlockSection>
            <apex:pageBlockButtons location="bottom">
                <apex:commandButton value="Save" action="{!save}"/>
            </apex:pageBlockButtons>
        </apex:pageBlock>
    </apex:form>
</apex:page>

5. Go to "setup/Create/Tabs" click on "new" under group "Visualforce Tabs"
6. Select "Visualforce Page" created before & provide other information & save.

7. Browse your custom tab to access the custom tab.

How to make a post or get request to some other server from apex class

1. Go to "setup/Develop/Apex Class"
2. Click "new"
3. Write code as following:

public class ApexClass_1 {
    @Future(callout=true)
    public static void c1(String Account_ID) {
        String name = '';
        try {
            HttpRequest req = new HttpRequest();
            req.setEndpoint('http://www.yahoo.com');
            req.setMethod('POST');
            req.setBody('Account_ID='+EncodingUtil.urlEncode(Account_ID, 'UTF-8')+'&other_param='+EncodingUtil.urlEncode('OTHER PARAM VALUE', 'UTF-8'));
            req.setCompressed(true);

            /* If you want to send basic authentication */
            String username = 'myname';
            String password = 'mypwd';
            Blob headerValue = Blob.valueOf(username + ':' + password);
            String authorizationHeader = 'BASIC ' + EncodingUtil.base64Encode(headerValue);
            req.setHeader('Authorization', authorizationHeader);

            Http http = new Http();
            HTTPResponse res = http.send(req);
            name = 'Status_Code=' + res.getStatusCode();
            String responseBody = res.getBody();
        }
        catch(System.CalloutException e) {
            System.debug('Callout error: '+ e);
            name = ('Error=' + e.getMessage()).substring(0, 20);
        }

        /* Reading account object */
        Account account = [SELECT Id FROM Account WHERE Id = :Account_ID];
        account.Name = name;
        update op;
    }
}

4. Go to "setup/Security Controls/Remote Site Settings"
5. Click "New remote site" & enter the URL that you want to invoke from apex class.
6. Invoke method "ApexClass_1.c1" from wherever you want.

7. An example of invoke url in salesforce from apex sObjects trigger options
8. Go to "setup/Customize/Accounts/Triggers"
9. Click "new"
10. Write the following code:

trigger TriggerActionName on Account (before insert, after insert, after delete) {
    if (Trigger.isInsert) {
        if (Trigger.isAfter) {
            for (Account a : Trigger.New) {
                ApexClass_1.c1(a.Id);
            }
        }
    }
}

11. Once you create a new account the url would be invoked in a short time (Also you could write code for update & delete)
12. Apex trigger documentation: https://developer.salesforce.com/trailhead/en/apex_triggers/apex_triggers_intro