Textbox Issues

I need to convert the text entered in a text box like:

this is a sample test

to

this_is_a_sample_test


Also in another textbox I need to convert the text to HTML so every time the user goes to a new line it needs to detect that, So what I mean is

This
is
a
sample


should be

This
<br>
is
<br>
a
<br>
sample


Any help would be appreciated.