how to find a word in richtextbox

i am using this code
<pre lang="c#">
if (txtsearchgerman.Text.Length > 0)
{
startindex = FindMyText(txtsearchgerman.Text.Trim(), start, txtshowfile.Text.Length);
// If string was found in the RichTextBox, highlight it
if (startindex >= 0)
{
// Set the highlight color as red
//txtshowfile.SelectionColor = Color.Red;
txtshowfile.SelectionBackColor = Color.Red;
// Find the end index. End Index = number of characters in textbox
int endindex = txtsearchgerman.Text.Length;
// Highlight the search string
txtshowfile.Select(startindex, endindex);
// mark the start position after the position of
// last search string
start = startindex + endindex;
}

public int FindMyText(string TxtToSearch, int searchStart, int searchEnd)
{
// Unselect the previously searched string
if (searchStart > 0 && searchEnd > 0 && indexofsearchtext >= 0)
{
txtshowfile.Undo();
//txtenglishtext.Undo();
}
// Set the return value to -1 by default.
int retvalue = -1;
// A valid starting index should be specified.
// if indexOfSearchText = -1, the end of search
if(searchStart >= 0 && indexofsearchtext >=0)
{
// A valid ending index
if (searchEnd > searchStart || searchEnd == -1)
{
// Find the position of search string in RichTextBox
indexofsearchtext = txtshowfile.Find(TxtToSearch, searchStart, searchEnd, RichTextBoxFinds.None);
//indexofsearchtext = txtenglishtext.Find(TxtToSearch,searchStart,searchEnd,RichTextBoxFinds.None);
// Determine whether the text was found in richTextBox1.
if (indexofsearchtext != -1)
{
// Return the index to the specified search text.
retvalue = indexofsearchtext;
}
}
}
return retvalue;
}
</pre>

it works but now i want to search sentence by sentences and also count the word how many times occurs.