Okay this might seem like strange advice but this is what I suggest. First get yourself a book and pencil for some planning before writing your actual script. I find planning to be helpful and just jumping in into writing the script can make the whole process much harder. So sit down, relax and start writing down the major points you intend to cover in the video. This does not need to be in order and I'd suggest you write down everything you feel is relevant. Then ask yourself, what's the tonal intent of the video. Do you want to sound humorous? Do you want to sound sullen? These are things to consider. Once you've got your points down and know the overall "feel" of the video, start grouping and elaborating on your points. Which point is relevant to each other, how should the points flow etc. Also, after that, get rid of the points you don't think is that important for your video. Now take your points and create a mindmap. Elaborate on each point, what is it you want do discuss in that point, what is it you want to convey etc.
Now that the planning is done, you are ready to write your script. Honestly, the reading a book thing will just get better with time as you get more comfortable. Also, saying you want to use fancy words and such isn't really something that can be helped as that requires practice. But don't fret! If you continue to actually take time writing your scripts and fleshing them out as best as you can, your writing ability will increase.
