Comfort and action
One of the major things to look at is the ability to sit comfortably with the guitar with a nice neck and action to play; this means that you can play the guitar naturally and that the strings will be close enough to the ret board that you don't have trouble pressing down a larger distance. Most really cool looking pointy guitars such as 'Flying V' and 'Randy Rhoads' shaped guitars are excellent to show off on stage but they are a real pain to sit comfortably with and you end up playing them with the neck and angled high or the fret board away from you making it a lot harder to play.
The best shaped guitars to sit with will have a cut out in the body that allows it to sit on your knee and the usual timeless 'Fender Telecaster', 'Fender Statocaster' or 'Gibson Les Paul' being the most comfortable and helpful shapes.
Different styles of guitars have different sounds and this can really cater towards the style of music you are playing. Choosing the style different to what you want to play can have you second guessing yourself and wondering "why doesn't this sound like the original?" With this in mind there are a few basic things you can look at when choosing a sound...
Different pick-ups give your guitar a different sound and depending on what you want to play this can greatly effect your end product. pick-ups come in what we call either single coil pick ups or double coil 'humbucker' pick-ups (pictured below). Hum buckers offer a very full bodied warm sound and can generally be a great place to start as it offers you such a full tone that you can feel what you are playing. The 'Gibson Les Paul' guitar has 2 hum bucker pick-ups.
Other options are single coil pick ups (it looks like half a humbucker) and these offer a very different and very bright sound. These are incredibly popular in country music lead guitar along with some modern indie music. A 'Fender Telecaster' is an example of a 2x single coil pick-up guitar.
Finally you can have a mixture of the sounds with either 3 single coil pick-ups or a hum bucker and 2 single coils. 'Fender Stratocaster' guitars are the classic design here and again give you a versatile and clean sound that allows a nice ring through the notes without it being too full or too sharp. Guitarists from Eric Clapton to Dave Murray of Iron Maiden use these for the control that you get over the sound.
Ok, so with all this in mind, more than likely budget will be the biggest factor deciding which guitar you get. Although above I have mentioned Fender and Gibson, there are many other brands that are a lot cheaper that offer affordable alternatives that are in the same 'Les Paul, 'Telecaster' or 'Stratocaster' style. The information regarding comfort and sound will be the same with these lines.
My best advice to you is to get to a local music shop and actually sit with the guitars and see which feels the best to you in the style you are looking for.
Finally and most importantly, whichever guitar you get, get playing and have fun!!