Go to your Autostart folder, in case of default Autostart path you should just execute:
Create a new file withing this folder (the name doesn't matter) using any editor, for example you can execute: vi start.script
The first line of your new file needs to be
This makes the file a script (you will make it executable later).
Now in each line specify what do you want KDE to execute each time it starts up. For example if we you want to start ktorrent every time, add the command into this file:
After putting all lines save changes to this file and exit
The last thing you need to do is make this script executable, so just execute:
chmod +x start.script
At this moment you may reboot KDE to see the changes during startup. Please take a note that this only affects current user.