Hello. I was wondering how to thread a blunt-ended yarn needle. Thanks!
Are you referring to casting on so you can begin to knit? If so, there are videos here that demonstrate several ways. A lot of people start with the long-tail cast on which seems really complex at first, but is pretty easy once you get the hang of it.
The videos are here.
Are you talking about the needle you use to weave in the ends? It is called a tapestry needle 
fold the end of your yarn and put the “fold” through the eye of the needle, if it won’t go through, you need to get a bigger needle !
EDIT–d’oh! I just saw in your signature that you already know how to cast on, so I’m assuming you are talking about threading a needle to do seaming…
If so, you do it basically like you would a sewing needle. You need to make sure the eye of your needle is big enough to accommodate your yarn.
The way I found the easiest to do it is:
Fold the yarn you will use for seaming over the top of your needle, and pull it down of the length of the needle to make a “crease” in the yarn. Then you poke the folded part through the eye of the needle. Pull one side of the folded strand through until your needle is on the yarn. I don’t think I’m explaining it very well, so hopefully it makes sense.
Marria:
I am reffering to the actual yarn needle. I got the name wrong, sorry! Could you possibly help me still?
If you can’t thread it the way you would a sewing needle, you may need one with a larger eye. Another thing is to wet the end of the yarn and twist it tight to make it smaller. I usually use crochet hooks to pull the tail through the stitches and for seaming.
I made another post up above–I hope that helped!
Here is a video that might help you see how to thread your yarn needle. Watch carefully as she folds the yarn over the large eyed needle. This makes it easier to insert into the needles eye. Hope this will helps answer your question.