iPad is going to be released and shipped on April 3 but if you can’t wait that long or wanna reserve an iPad ( in-case they run out of stock) then you could pre order it.
The pre orders for the iPad start at 8:30am EST Friday.( 5.30 am , Pacific).
UPDATE – Apple now accepts pre-orders. You could go to apple.com/ipad and pre-order one right away. The WiFi only version will be delivered at your doorstep on April 3, but the WiFi + 3G versions would be available late April. Or you could reserve an iPad – select a store near you and Apple will reserve one for you. Then you could visit that store on April 3 and get your hands on it.