Newbie question: How can i load troops?

Before your transport is launched (don't auto-launch them!), select the star-system that it is sitting in, look for the transport icon in the "Ships in orbit" window on the bottom-right of the screen.
Double-click on the transport, it should ask you how many warriors to put aboard before launching.
The same can be done with colony ships which will otherwise take exactly half of your population with them.

And that's half of your *system* population, not the planet that built it.

~SDC~

Thanks