Thin plywood reinforced with fiberglass and a top rail made from spruce is cheap, easy, and lightweight. Using this method, my husband and I made a rowboat, he made a ~16ft pirogue (sold 9 years ago), and my son just completed a 13 ft pirogue (a type of flat-bottomed canoe).

The attached picture is the new pirogue during painting. It looks much nicer now that it is complete.

Google [easy boat plans]

The first boat you build will be a little lumpy, but that's to be expected. It's all good as long as it floats and you will learn a lot from the process.

As is typical for a question with few specifics, the answer is, "it depends."
I'm going to assume you're referring to a generic plank side type dinghy or row boat and not a sail boat, canoe, or other vessel.
For the structure use oak or a similar type hardwood; for the planking you can use cedar, spruce, poplar or generally, whatever is readily available where you live.

What kind of boat makes a big difference.

Most kinds from canoes to motor boats can be built with plywood, often 6mm thick.

Have you looked at any boat plans?

http://hallman.org/bolger/isometrics

http://gaboats.com/boats/classic10.html

http://koti.kapsi.fi/hvartial/stab/stab.htm

http://www.woodenwidget.com/index.html

http://www.selway-fisher.com/Stitch%20and%20Tape.htm

http://stillwaterboats.com/

The cheapest wood you can find!?

It doesn't matter what kind of wood you use. What matters is how you finish it and seal it. But some woods like teak or ceder are better suited for a marine environment because of there natural weather resistant properties. You'll find those options are much more expensive then some cheap pine.