You will want to use the Type.GetType method.
Here is a very simple example:
using System;
using System.Reflection;
class Program
{
    static void Main()
    {
        Type t = Type.GetType("Foo");
        MethodInfo method 
             = t.GetMethod("Bar", BindingFlags.Static | BindingFlags.Public);
        method.Invoke(null, null);
    }
}
class Foo
{
    public static void Bar()
    {
        Console.WriteLine("Bar");
    }
}
I say simple because it is very easy to find a type this way that is internal to the same assembly.  Please see Jon's answer for a more thorough explanation as to what you will need to know about that.  Once you have retrieved the type my example shows you how to invoke the method.