You don't give us a lot of details to work on, so I'm going to give you a broad answer.
First of all, I'm going to assume that you are actually using a correct version of OS X. 10.5(Leopard) is the most recent version supported by any Powerbook G4(or any PowerPC Mac). All will support Tiger, and for some Powerbooks this is the only version officially supported.
The next step is to be sure you're actually making the USB installer correctly. I was advised by a knowledgeable member here to use Disk Utility to format the USB disk as Mac OS Extended/APM, mount the .dmg or .iso of the disk image, and then use a program like Carbon Copy Cloner to clone the mounted image onto the USB drive. I was able to create a good install stick by doing this.
The next step is booting. If you have a Titanium Powerbook, you can probably hold down "option" while booting(after it chimes) and a blue screen with all bootable volumes will appear. You can select your USB install stick from here. Just be aware that it will be slow.
If you have an Aluminum Powerbook, you will probably need need to boot into Open Firmware by holding down

+option+O+F during startup(before the chime). This will boot to a grey screen with a command prompt.
I was advised on here that, from the open firmware command prompt, you can usually type boot
ud:,\\:tbxi and it will boot off the USB. It's worked for me every time I've tried it, at least.