Hello Gary
Do you have any mates that already play guitar that could maybe come with you to the shops? I would say that there are 4 different criteria to look at:
1) How it looks
2) how it sounds
3) how it feels to play
4) what it costs
You can decide for yourself on 1 and 4. You may be able to decide on 2, although I have found over lots of years of playing, and owning quite a few guitars, that my "ear" for what tone I like from the guitar changes from time to time. It's kind of like human voices. One week you might like to hear, for instance, a choir singing in crystal clear voices. But another week you might like to listen to a blues singer, or a rock singer with a harsh rasping voice. To some extent, it would be fair to say that all guitars each have one basic "voice" and the trick is to match the type of guitar tone to the type of music you want to play.
To some extent, you can modify the tone of a guitar through your choice of strings, and through your playing style, but this will all take time to learn. However, if you have a mate who plays, he or she might be able to help you match the inherent tone of the guitar to the type of music that you aspire to play.
Also, they can perhaps give you some input about how the guitar feels to play (point 3 of my list). Are the strings easy to hold down? Are the ends of the frets nice and smooth or do they catch your fingers anywhere? How close together are the strings? Are they too much of a stretch for you, or are they too closely spaced and leave your hand cramped up?
As regards makes of guitar, Yamaha are pretty good, but the really cheapest ones have the top (the bit with the soundhole cut out of it) made from a "laminate" of thin sheets of wood glued together. Most guitarists would say that the sound is not as good as if the top of the guitar is made of solid wood. But of course, the sound of a laminate topped guitar might be what your ears tell you that you want... it's just my opinion that I probably wouldn't choose one for myself.
Another make which comes out well in reviews is "Vintage" who make a huge range of guitars from about £120 and upwards. Well worth considering.
Two final points:
1) The strings on the guitar have a huge influence on the sound of it. Often in guitar shops, the instruments are still fitted with the strings that were put on at the factory(especially with the lower priced guitars), and the strings can go "dead" especially if the guitar has been in the shop a long time, and/or has been played by lots of people. Changing strings will almost certainly alter the tone of the guitar significantly. Usually it will make it clearer and brighter, kind of as though you had cotton wool in your ears and then took it out and everything sounds that bit more clear. But there are some shops that fit new strings to all the guitars that they put out for sale, so ask about this when trying guitars.
2) The "set-up" of the guitar. A skilled guitar technician can adjust how far above the fingerboard the strings are placed. The closer to the fingerboard, the easier the strings are to hold down. But the more likely it will be that the strings rattle or buzz against any slightly high points on any of the frets. So it is a compromise to find string heights that are right for you.
I only mention this because some music shops just sell guitars straight out of the box, whilst others give each gutar a "set-up" by a skilled person before they turn it loose into the shop. Ask about this when trying guitars, and don't try to make any adjustments to the guitar for yourself, until you are 100% confident that you know what you are doing.
Best of luck with starting out. The guitar is a great instrument, and even learning a few basic chords can allow you to bash out a tune, which will impress the hell out of all the non-guitarists that you meet!