Ok its a bit hard with your criteria, Im guessing you have access to sharepoint central admin?
If not dont worry! you already have everything ;) except for one other thing that will make your life easier! WSPBuilder: http://wspbuilder.codeplex.com/
Install that, its like an addon to vs2008, you can build and deploy wsp without the need to use stsadm or central admin to add it to the server.
The next part is to make the webpart:
This is a basic example in sharepoint 2003 what is similar if not the same as 2007 and 2010. Making the controls and logic goes into creatChildControls.
build and deploy webpart using WSPBuilder.
Adding the webpart to the webpage:
click on "Adding Web Parts to a Web Part Page" in contents
using wspBuilder means you dont need to access the server as it adds and deploys it for you, you just need to do it on the sharepoint site like adding the webpart.
That is pritty much it hope this helps :)