For a first attempt or two I'd be tempted to go to the local timber merchant & get a decent piece of white ash. You'll not make a superb or high poundage longbow, but you'll learn a lot about how to do it (& how not to) along the way. They're just right for middle sized kids too. £20 gets you enough for 3 bows
Right at the beginning of my journal are some pics of the last one I made.