Page in Admin page

How to create another page in admin page.

I have created admin page and in this page i have created a few links how can i create another page to open via those links?

I’ve tried with $_GET[‘page’]

main page admin.php?page=tasks

I created link admin.php?page=opentask and tried catch with

if($_GET[‘page’] == ‘opentask’) {
echo “Hello world!”;
}

but i have error:
You do not have sufficient permissions to access this page.

How can i solve this problem?

Thanks