One skein of the main color will be more than enough for the hat. Use odds & ends for the ornaments. This would be a great pattern for a beginner. Skills used are cast on, knit, purl, decrease, mattress stitch, duplicate stitch, and make bobbles, and links in the pattern lead to tutorials, in case any of the techniques are new to you. If you use a softer yarn, perhaps a worsted-weight cotton or merino held double, you won’t need to line it. Do knit a little stiffly, however, to make the top of the tree stand up.