There are a confusing number of choices... First, if your phone has miracast, you can just mirror it, so investigate that first - whether you phone supports it and how to activate it.
Next, there are better ways to do most things unless you want to display your text screens or something else completely unique to your phone.
There is a roku app that you can install on your phone that works as a remote control but can also send content from your phone to the roku. This isn't mirroring, but is better for music, photos, and video files that are on your phone.
If you intend to play content from the internet on your phone and mirror it to the TV, it is generally better to find a native roku app/channel that will play it directly. Since you mention youtube, yes roku has a native youtube channel and you can either select the content with the remote or use your phone to search, then cast a video to the TV. If you touch the little screen-looking cast icon in the phone youtube app you should see the roku as a possible destination. Netflix will do this too, but most others won't. For example if you have Amazon video you have to select it with the roku remote, but you could use the phone to add something to the 'watch next' list to make it easy to find.
