You can practice picking up the sts - cast on about 12, work a few rows, then bind off. You can pick up sts down the side of the rows, then also along the cast on edge, up the other side and across the BO edge. That ought to help you get the idea.
Top down raglans are fairly easy - there's no seaming and you'd only have to pick up sts around the neckline. There's some pattern generators a www.knittingfool.com and woolworks. These give you a pattern based on your gauge and the finished measurement you want.