I just started knitting recently, too! And with a LOT of help from these videos as well.
First may I suggest size 7 needles for that yarn, you may need to go down to 5 or up to 9 depending on how tightly or loosely you knit, but that sounds like a nice middle size (at least for me).
The next thing to consider, something that I didn’t really think about and ended up frustrating me, is to make sure you have enough yarn to finish your project!
My next piece of advice is that you shouldn’t think about doing the scarf yet, especially if the yarn is specially bought/rare. I think it may be a better idea to do all your test runs and practice knitting with a cheap store bought yarn in the same weight. Then, make gauge swatches of both of the patterns you posted. It not only will give you a better idea of how tight you knit, but you can have a little sample of the actual pattern (you might like the backside of one kind of rib over the other, or like a border or not, or may find that one particular pattern tends to curl a lot more than another, etc etc). Also, making small samples will give you practice not only DOING the actual stitches, but learning to recognize what a purl stitch and a knit stitch looks like. My knitting life became SOOO much easier when that lightbulb finally went off and it all clicked for me. I’ve read and heard it said a lot: “If you imagine the loop on your needle as a guy’s head, and look at the ‘neck’ and the stitch right under it, the purl stitch looks like a guy wearing a scarf, while the knit stitch looks like a guy with a noose!”
sorta like this
/ \ purl
Ok, I dunno if that came out nearly as good as it did when I thought of it. Watch the video of the “small demo” that’s basically the idea.
When you can look at a stitch and recognize what it is, you’ll become so much faster and won’t make nearly as many mistakes by knitting when you should’ve purled or whatever.
Then, you can choose which pattern you like and roughly figure out if you’ll have enough yarn to do your scarf and then go for it! The scarf itself will not be nearly as frustrating if you take the time to make small swatches as practice. And it’ll look a heck of a lot better too!
I hope this helps!