Pretty much any computer will do. I'd recommend a cheap Windows laptop, because I like coding from the couch. If you don't care about portability, a cheap desktop will do fine.

So you need something that can run at best SNES style games... I would say a Dell from Walmart will do! Haha jk I would never ever ever recommend one of those. Sounds like you just need a decent machine since you won't be doing compiling or at least minimal not sure how RPGMaker works. I do however recommend building a machine you will always get more bang for you buck. You can order a bare bones and a decent graphics card probably spend like 400 - 600 and call it a day. If you want a little better you can up the cost and go to 800 - 1000. Newegg is a good place to start.

The best thing is you pick what you want and link to it and people will always give you help and let you know if that works out recommend something else to assist you.

Decent screen ( this means good resolution too, coding on a 768p is a pain in the ass)

Decent keyboard

... all the rest should just capable to run the OS.

For your needs, just about any off-the-shelf computer will do. The only thing I would really look out for, in a desktop, is to do some research and make sure you get a standard ATX power supply, rather than something proprietary like some of the less-expensive manufacturers use. The reason is that often times these proprietary units are low-wattage and lack the additional power connectors that even some lower-end discrete GPUs require. If its a standard size and layout, at least you can replace it, should you need to. Luckily proprietary PSUs are less common than they once were, but you still have to be aware of them. Double-lucky, many mainstream OEM-style PSUs will support enough additional GPU power connectors to drive at least one middle-of-the-road GPU.

Other than that, shoot for at least 8GB of RAM, and get a decent SSD if you can afford the luxury -- even a smallish (128 GB) one if its all you can afford. Both are relatively small costs, but are the best thing you can do to increase your computer's perceived performance. Often times, when we feel that our computers are "being slow" its because its waiting on the slowest componenent in the system, which is the hard disk, either because you're accessing files, or because you're using more RAM than your computer has and the OS is swapping pages of memory to hard disk.

For myself, I usually build from scratch or have bought a base workstation (as in, a computer for serious work, not a consumer PC) and made the upgrades I wanted. If you're feeling adventurous and maybe have a technical friend that can help you out, a self-build might be worthwhile if you have a local place you can get components for fair prices. Newegg is great too, but it's a lot more hassle to deal with if you happen to get a faulty component, so I don't recommend it for an inexperienced builder.

If you're in the market for a laptop, the biggest thing is getting a good-quality screen that's a comfortable size and resolution. Like Alessio said, 1366x768 is miserable to work on -- 1600x900 is passible, but 1080p (1920x1080) is better. For me, a 14" screen is the ideal compromise of screen-size vs computer size, but right now I've got a 15.6" laptop (1080p) because I wanted hardware you really can't find in a smaller machine today, let alone two years ago when I bought it. You'll also find some very-high-resolution displays nowadays -- they're nice because you get really sharp text and images, but at small physical screen sizes more pixels doesn't translate to more useable desktop space because the user interface becomes intractably small; in terms of usable space, cut those high-resolutions by a factor of 2/3rds or 1/2 to get a sense of how much usable workspace you get. You'll also want to be wary that you can't add more RAM to some laptops today because its soldered in to save space (mostly a problem on very thin computers, and on all Apple computers regardless of size), you have to buy the amount you want when you buy it and be content with it. The next biggest factor is the keyboard and pointing device -- you can use an external keyboard and mouse (and display) at home, but you won't have that at the coffee shop, and even just a mouse is a no-go in cramped spaces.

Get any off the shelf pc, add some extra ram and get a second monitor.

The extra ram is so you can easily run a paint program like gimp or photoshop while running you dev environment (RPG maker) sound editing tools and an instance of your game, plus Web browser etc.

The extra monitor is wonderful to have, and once you've tried it you won't go back. Use them effectively, putting your game on one and your dev environment on the other, or whatever floats your boat.

I tend to have visual studio on my 24" monitor and my game and a Web browser on my 17" laptop screen.

Anything that meets the RPG Maker system requirements will do. :)

If you do decide on a laptop I would personally recommend that unless you're planning to use an external keyboard you look for one with a full-sized keyboard rather than the cramped versions many laptops have -- this is really personal preference though, and many people are perfectly fine without it, so take that advice with a grain of salt and be sure to consider your own preferences. You'll also want a nice monitor that isn't too small or overly low resolution.

Plenty of RAM (you want to exceed the recommended amount for RPG Maker if possible so you can run other software at the same time smoothly) and an SSD are desirable for nice snappy performance.

I know you don't need it at the moment and perhaps a bit pricey for what you want to achieve right now, however I find it helpful using a MacBook with Bootcamp for dual booting into Windows also. This allows me to develop and test games for Windows, Mac and iOS (and Android).