Plug it in and wait until the orange light turns green, then try to turn it on.
If I remember rightly, the orange light is the battery charge indicator. The battery needs to have some power in it to boot, even when attached to the mains. If it doesn't turn green or never boots, the battery might be in need of replacement.
Just out of interest, it's a bit long in the tooth now, what are you planning to do with it? It might struggle with more recent software requirements.
BW, you can download a manual here:
http://www-307.ibm.com/pc/support/si...AN-3TJKLN.html