c# alter url, read data from a site then check the data inside

I'm messing around with WinForms and am stuck. I'm looking for a way to read the site, the site is basically text and should be easy to store in a variable, but how good would that be if the string is very long and how would I draw from it?

To explain further, my three questions are: 1. I first need to alter the website url using user input. Example; we have the base website http://ift.tt/2HgQeDD, how would I go about changing that with user input? Should I store the user input then just add it into the WebClient like this?

System.Net.WebClient wc = new System.Net.WebClient();
string webData = wc.DownloadString("http://base.com/" + userinputstring);

2. I need to read the text from the site and store it somewhere, can I store it using the previous code example? Could I do the third question then?

3. I need to read the stored string from the site and base my code off it. Example; if the string contains the keywords "xyz" the code would do something.

I tried to explain it as best as I can and I hope it's at least a bit understandable.