Hello everyone! I am looking to buy a new acoustic guitar. The main one that I am currently looking at is the Martin D-18. But to be honest I have no idea what I am doing. I don't know size differences like 3/4th or dreadnought or anything like that. I do know that I am comfortable with anything above a 3/4th size. I am willing to pay up to 2500 USD for my new guitar. I would like it to be a martin unless you can convince me otherwise. Other than all of that I don't know what I am doing. I hope you all can help me out!
If your spending that kind of money on a new guitar you really need to go to a music store (or several) and try guitars out in person. Try out a few Martins, Gibson's, Taylor's, etc. find one that you feel comfortable with and enjoy the sound of. A quick review of two basic guitar shapes are as follows:

Dreadnought: Original Martin design ("thee" acoustic guitar shape), fairly large bodied, excellent for flat-picking and scenarios where you are playing with other instruments, cuts through the mix really well. Suited for bluegrass, singer-songwriter, pretty much any style of music.

000: Probably the second most popular style, body is smaller and not as wide as a dreadnought, most players find these guitars very comfortable, doesn't' have the deep bass of a dreadnought, very balanced tone. Suited for finger-style (the fret-board on some models is wider), blues, folk, again a Swiss-army knife of an acoustic guitar.

They're other styles including Jumbo, Slope-Shoulder, 00, 0, parlor, Advanced-Jumbo, etc. Many manufacturers make several of each style. Get out there and try out as many as you can, although the D-18 is a fantastic guitar.
i would definitely suggest you wait to spend that much till you've tried enough guitars to discover your preference in nut width, neck profile and tone. i bought a bunch of guitars till i discovered that 1 11/16" nuts - the most common width - were uncomfortably thin for me. i ended up selling all my guitars and starting over.

you may also discover you prefer brighter guitars - martins aren't generally bright guitars.

if you can play enough to get to a guitar store and try everything, then do it again a few times, you can make note of what necks and body shapes feel the best and go from there.
Quote by Skeet UK
I just looked in my Oxford English Dictionary and under "Acoustic Guitar", there was your Avatar and an email address!
You really need to play them. Martin is the Godfather of Acoustic guitars but it all comes down to what you like, what feels best to you, ya know? And Even when you get into Martin's you need to play as many as possible, like a D-28 and HD-28 will not sound the same even though they are the same woods. Some have different Bracing. I'm not trying to detour you but that is serious cash to be dropping a guitar that will sit in it's case because you hate it or don'r get inspired by it.

It took me 5 years of playing before I really knew what I liked. The only problem is I'm poor. lol