How can I safely handle POST parameters in an HTTP Handler using C#?

How can I safely handle POST parameters in an HTTP Handler using C#?

I'm working on an ASP.Net C# application (my first!) that contains an HTTP
Handler within it. My application works with several parameters that are
passed in via the URL. My question(s) is as follows:
My applications entry point is via the HTTP Handler. When I enter my
ProcessRequest method, I am assigning the values of the URL parameters to
variables so that I may do something with the data. Question: Is this
safe, even if I am not setting the value to anything when I call the URL?
Example: I call host/handler1.ashx instead of
host/handler1.ashx?something=foo
public void ProcessRequest(HttpContext context)
{
context.Response.ContentType = "text/plain";
string something = context.Request["something"];
context.Response.Write("Hello World: " + something);
}
When calling the above method using the plain URL with no parameters, it
executes just fine, but the string something is just blank/null.
Additional questions: What happens to the variable something in the case
that I do not explicitly initialize it via the URL? I understand that it
is null, but can this lead to problems?
Is it dangerous or not safe to call the plain URL (i.e. should I always
call it with parameter values specified)?
What is the best way to call a "clean" ashx URL to start the application
but not risk problems?
The application will do a series of subsequent GET redirects as it
accumulates values and passes them back to the app via the query string.
Should I do a POST or GET upon initial call of the application?
Sorry for asking the same question multiple ways, but I'm a bit confused
on the topic and this is my first time writing an app like this. Any
patience and advice you could provide on how to safely handle and
initialize parameters is greatly appreciated!