You can use this method to just to create a text file.
File.Create("D:\\Yourfile.txt");
for that you need to use this namespace
using System.IO;
If you need to read and write to a file use below code. It will do the work.
FileStream fs1 = new FileStream("D:\\Yourfile.txt", FileMode.OpenOrCreate, FileAccess.Write);
StreamWriter writer = new StreamWriter(fs1);
writer.Write("Hello Welcome");
writer.Close();
FileStream fs2 = new FileStream("D:\\Yourfile.txt", FileMode.OpenOrCreate, FileAccess.Read);
StreamReader reader = new StreamReader(fs2);
textBox1.Text = reader.ReadToEnd();
reader.Close();