how do i make a software auto start after login?
 

such like gaim? or Ksim? thank you very much!

Depends if you login in graphical or text login. I login as text, so then I edit the file called xinitrc (it could be either located at your /etc/X11/xinitrc or at your home directory as .xinitrc

There, I search after the line which start my display manager and add what I want to start too. E.g:

exec gaim &
exec blackbox

That would start gaim, and then blackbox. Remember to add an "&" at the end of all programs you want to start, but not the last one ;)

If you're using KDE, there's a ~/.kde/Autostart directory....I don't use KDE so not sure what you put in it....I assume probably a symlink

If you use Fluxbox, 0.9.6 or newer, you can add a line to ~/.fluxbox/apps:I don't really like adding applications to ~/.xinitrc since some apps don't work well in some window managers.