First let me say that Cottenwood is 110% correct about the needing feathers!
Now as far as finding the right spine for arrows there are a couple different means of getting to the same end point.
First you need to choose the type of shaft you want to shoot. I would recommend for any beginner to go with carbon because they are 100% straight and you don't got that with wood, never.
Next you need to select a carbon shaft in the weight range for you bow. If the weight is 50 lbs. at 28 inches but you draw 29 inches then your draw weight will be 52 lbs. I would recommend a carbon in the 150/400/3555 range (depending on the manufacturer the spining is designated differently). If your draw weight is 55 you need a 250/300/5575 and if it is 45 you need a 90/500/3555.
Next decide how long you want your arrows. The shorter the arrow the stiffer the spine becomes. So if you cut a 150 down to 25 inches it is now in the 250 range. I would recommend not to cut an arrow any shorter than 29 inches unless a good Arrowsmith is involved in the arrow making (not a bow shop or 3rivers or Bass Pro, but a good Arrowsmith.
Next order a field point test kit from 3rivers that gives you 2 heads from 100 to 225 grains.
Then you are ready to bare shaft tune. From 15 yards start with two shafts that have nock and insert installed. The put the 100 grain heads on them and shot them. Watch the arrow fly and make note of it. Move up in weight of your points and shoot again. Continue this until you have shot the 225 heads. Then the heads that flew the best bare shafted is the weight you want to continue to use.