I have a lap top and a desk-top that I want to hook up to one laser printer.Help me please

Is your laptop wireless and do you already have a wireless router employed? If so, you can get a print-server component which will share your laserjet through a router port with the connected PC and wirelessly with the laptop. Print servers come with their own wireless access point too as do routers with a print server port function built right in.

Another solution would be to connect the printer to the desktop and share it so that the laptop (again, assuming wirelessly connected) could use it; only works for the laptop when the desktop is powered on. To do this, from the desktop, once the printer is installed and configured, right-click on the icon for the printer where displayed in the Printers folder and select Share and go from there. Then after that, from the laptop, either search for it in your network neighborhood or use the UNC line for the device which would be \\{desktopname}\{sharename}.

Finally, you could just get a 'dumb' print sharing box; basically has an A-B switch (or ABCD for four shares, etc.) and reverse it such that the desktop can print to it (directly) when in switch A position and the laptop (again, directly) when in switch B mode. This really isn't much different from unplugging the printer cable from one computer and in to the other except is saves you, well, the actual physical unplugging.

As pchartwell pointed out you have a number of options. Which is the best option depends on your equipment.
How far apart are they normally? Are they networked already? If so how? What interfaces are available on the laser printer? What model is it?