Hi there!
To provide some backstory for this post - I'm a beginner who just got my first guitar yesterday and am super excited to start learning all about this wonderful world of guitar music. The guitar I got is a Rogue RA-090 Concert Body Acoustic-Electric guitar (link: http://www.guitarcenter.com/Rogue/RA-090-Concert-Cutaway-Acoustic-Electric-Guitar.gc).

I began tuning the guitar for the first time yesterday night, and it played great! I learned a few chords yesterday, and today I learned the D Major chord. When I tried to play it, however, there was a noticeable buzz from the B string. I tried changing the position of my finger around, seeing if pressing down lighter or harder at different positions on the 3rd fret's B string would make a difference, but no matter what I tried, the buzz was still noticeable.

I then tried each note on the 3rd fret individually, and the same thing happened. Every time I played any string with my hand anywhere on the 3rd fret, no matter what string I played, there was a noticeable buzz. I've tried changing the positioning of my fingers around and pressing lighter/harder, but nothing gets rid of the buzz!

Is this possible to fix? If so, is it better to ask the repair guy in the local guitar store or to try and fix it myself? (and what tools do I use?)

Thank you so much for reading!
-Kynnaryth, a beginner
Sounds like your 4th fret is high.

I'd look into returning this one and getting another one. Ask the store to tune and set it up for you and then play every fret to make sure it is acceptable before you leave the store.
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Kynnaryth Well, as "Metalmingee" said, it does sound like the 4th fret is too high. A possible cause is simply it not being set correctly at the factory.

But, "neck relief could also cause that symptom as low as the 3rd fret. What you should do, is to continue up the neck to every fret and see if the buzzing persists.

And BTW, it's just as easy to imagine the 3rd fret is too low. To determine the exact location of the problem, a high quality straight edge, and / or a tool called a "fret rocker" needs to be employed.

And no, I don't recommend a novice such as yourself goes wildly into attempting repairs. If this is a new guitar, a trip back to the store is in order.

Here is a great guide for setting up an acoustic guitar. I'm advising you not to go tearing into your new guitar, but I think you should take the time to learn the setup process. http://thbecker.net/guitar_playing/guitars_and_setup/setup_page_01.html
the fret could be high or popping out, could need a fret level, or a hammer and super glue.
If using a hammer you want to use a brass or aluminum block that covers 3, 4 frets.  Go up the whole fretboard.  Extra whack at the body join.  If the block starts getting dents shift it so they don't land on subsequent frets.  Brass or aluminum can bend so use a piece of steel plate on top of the block and a plastic faced hammer.  Make sure the body is padded on the bottom and the neck is supported under the fretboard.