if you make changes to aspx or web.config, you won't need to compile.
if you make changes to code-behind (i.e filename.aspx.cs) you will need to re-compile.
Depends on how big of changes you are talking about, if its very small, just make the changes, and change the timestamp of the folder which contains your .NET application, IIS will auto re-compile for you.
Otherwise, it maybe easier for you to bring the project up in VS.NET first, and re-compile from there.
It is also possible to compile by hand:
csc /t:library /r:System.dll /r:System.Web.dll Hello.cs
/t:library indicates that you want to create a dll
/r specifies any dependt assemblies
the output dll <b>MUST</b> match your Page directive otherwise aspx won't be able to locate your code-behind files...
if you are not sure what you are doing, I suggest you get VS.NET