I just watched a few YouTube vids on setting up a Roku box and on one the demonstrator said I need a "...TV or computer monitor with tuner..."
Just because some unofficial video said it, doesn't necessarily make it so...
I'm sorry to ask this again, I'm a little confused... is having something with s tuner necessary?
Only if you want over the air TV on the same display as the ROKU. The ROKU itself simply needs a compatible input you can hook it up to.
If the box is getting the signal from the internet, why do I need a tuner?
The only reason you would "need" a tuner, is if you also wanted the feature (over the air TV) that the tuner provides. I know for sure that a ROKU player can hook up to just a monitor (if you have the correct inputs on your monitor, and your monitor can handle the proper display resolution, and you get/purchase the optional cables needed to do the proper hookup), as I've done it in the past.
Before I got my 32" TV, I had hooked up my original model ROKU streaming player (the original model is no longer being made, but the newer models have similar hookup) to a 19" LCD ("Hannspree" brand) monitor by an HDMI cable (that particular monitor had a single HDMI input, along with the standard VGA/PC input). This was an actual monitor, not a TV (no tuner built in), but it did have the compatible HDMI input (which I used for the hookup). And this setup worked fine with the ROKU (albeit with the limitation of the smaller 19" screen size). The ROKU was very happy to see my monitor as a (720p) HD wide screen "TV", when hooked up this way.
FWIW: I still have that 19" monitor, I just no longer use it as a ROKU display, as I prefer the much bigger viewing size on our 32" (720p) LCD TV. Instead, my monitor has been moved into the "home office", and is in fact now hooked up to a laptop. Since I also have added a USB keyboard and USB trackball (mouse replacement), I am now using that laptop as if it was a low power "desktop" (in fact, I'm looking at that 19" monitor right now, as I type this stuff in).
So the bottom line is that if your monitor has the proper hookups, and can display the TV signals correctly, it can be used with a ROKU. Since you already have a DVD player and cable box hooked up, your monitor clearly has the proper hookups and display abilities (otherwise your monitor wouldn't work with the cable and DVD boxes). However, you will have to make sure you have a spare hookup spot on the monitor for the ROKU, or you will need to get a video switch (as another poster suggested). Also, the ROKU only ships with the most basic of "composite video" cables (and your monitor may very well not support composite video). So it will be your responsibility to find/buy the proper video (and audio) cables to hook your ROKU up to your monitor. Such cables are pretty easy/cheap to find, but it will still be an extra step (and expense) you will likely have to go through.