I assume you are using Poser 8 or 9.

What follows is the Cliff-notes version of War and Peace, as there are volumes of threads about this already. It is the simplist (and over symplified) version I can offer a new user.

IDL stands for "In-Direct Light" and is a mimic for the way light actually behaves.

Using it outside involves using a self-lit dome to mimic the sky in conjunction with a single infinite light to stand in for the sun. Bagginsbill offers a great dome here: https://sites.google.com/site/bagginsbill/free-stuff/environment-sphere for free.

Indoors, the plan most often used is to look and see where light is actually going to be coming from. If there are windows, put the dome ouside over the room set (but put a ceiling on top of the room). If the sun is shining directly into the room, you may have to amp the sun up to 600-800% to get a realistic effect. Put point lights wherever there is a lit lamp, and set them to inverse square falloff in the "Attenuation" drop-down of each light's properties window. 

Overall, lights in IDL only need to be set at 10-20% power as they get reflected and amplified by either the dome (ouside) or the walls (inside).

At render time, many suggest using D3D's Firefly render script located at: Scripts > Partners > Dimension3D > Render Firefly in your Poser install. I have attached a screen capture of the way I run it although many people will recommend other settings to accomplish specific goals. Their answers are "right" too.

If you want to speed things up a bit, set "precalculation scale" to about 50. The quality trade off is minimal.

These are the basics. Give it a try and report back your results and we'll go from there.