How to access sharepoint data using C#?

The SDK is a good place to start. The real crux of question lies in whether you are writing code which will live in a SharePoint environment, or writing code which will consume SharePoint data in an external application.

In the case of the former, SharePoint has its own API which you gain access to by simply referencing the appropriate DLL.

For the latter, SharePoint comes with a set of web services which allow external applications to consume its data. Either these or a set of custom services (running in the SharePoint environment) will be your entry point into SharePoint.

This is how you would do it in PowerShell which is very similar in how you would do it in in C#:

# Lets reference the assembly / GAC that we need for this
function getUsers
{
    param ([string] $verify_sitepath="https://extranet.something.com")
    $verify_site=new-object Microsoft.SharePoint.SPSite($verify_sitepath)
    	$verify_web=$verify_site.Rootweb
    $verify_web.site.url
    $verify_groups = $verify_web.groups | ? {$_.Name -match "^.*$CurrentGroup" }
    foreach($verify_group in $verify_groups)
    {
    	foreach($verify_user in $verify_group.users)
    	{
    		$verify_user = $verify_user -replace "WRKGRP\\",""
    		Write-Output "$verify_user" | Out-File -filepath "$splist$currentGroup.txt" -append
    	}
    }
}

What this does is gets all the users from SharePoint that are in a text file. Hopefully this gets you at least thinking about how SharePoint is set up.

A great resource is the MSDN page with all the functions. They provide a lot of programming samples in C#!

You have to install VS 2005 or VS 2008 extensions for sharepoint. Intsllaing them on xp can be tricky and this page should hep you with that.

Start at the Sharepoint SDK page. Download the SDK, and look at the sample code on MSDN.

Added later: according to MS, this is a better site for all things related to Sharepoint development.

you should also CAML Query which you should know to query data from sharepoint lists
you can make use of such a tool http://www.u2u.be/Res/Tools/CamlQueryBuilder.aspx

To me it sounds like you should use the Out Of The Box SharePoint web services. There is no reason why you should have to learn the entire SharePoint API when you could get along just talking to the web service.

This primer on InfoQ is good, but do a seach on SharePoint Web Services and you will find plenty of sources