Before creating site collections, you can read how to create a web application in this tutorial. You can see this article if you want to create a site using SharePoint designer.
Site collection is a hierarchy of sites having one top level site with a collection of child sites below it,the top level site is called as Parent site.

Site collection is a hierarchy of sites having one top level site with a collection of child sites below it,the top level site is called as Parent site.
Thease are the steps to create a site collection
Step-1:
Go to Start->All Programs->Microsoft SharePoint 2010 Products->SharePoint 2010 Central Administration.

Step-3:
Then you can use the default web application or else you can cahange the web application by clicking web application dropdown ->Change Web Application.