During the course of a walk, your feet will take a lot of strain through the constant impact of your body weight against the ground. The job of your walking boots is to help your feet (and ankles) to support this force, and hopefully reduce the amount they ache afterwards. Walking boots provide a lot more support than trainers, especially around the ankles - if you twist an ankle on the top of the moors it will hurt, and if the rest of the group misses their dinner because they had to wait for the Mountain Rescue, you won't win yourself any friends. A decent pair of walking boots should also keep your feet dry, which is always a good thing! This is a brief guide on how to choose a pair of boots that will work well for you.
It is important that your boots are quite a tight fit, but not too tight or they will pinch your feet when they bend. If they are too loose, your feet will move and rub against the leather, this is the cause of blisters. Remember that when walking you will probably be wearing thick socks, maybe two pairs, so do the same when trying boots on (shops usually have spare pairs for this). To test for general tightness, tie the laces, you should have room to wiggle your toes. Now push your foot forwards in the boot, there should just be room to insert a finger between your heel and the boot. On walks, you will be on steeper terrain than usual, this affects the way your feet move with your boots; pushing towards the front of the boots on downhills and bending much more than usual on uphills. Some outdoor shops have a ramp to test the boots on steep slopes. Facing downhill, stamp hard against the ramp. The heel should be held firmly in place so that your toes don't hit the front of the boot. If they do, they will rub against the boot and may blister on a long downhill, try a smaller size (or a different boot if the smaller size is too tight). Now face uphill and push off forwards, the boot should be able to bend at the toe and ankle without squeezing your foot.
The main reason why the club insists on walking boots is for the support they give to your ankles. On rough terrain, it would be easy to twist an ankle if you didn't have the stiffening effect provided by boots. The other major supporting element in the boot is the sole. Walking boots have much stiffer soles than trainers. Stiff soles help you to stand on small rocks and help the muscles of the feet to transmit power to the ground. With such stiff soles, you may need some cushioning to lessen the impact on your feet. Most boots have internal cushioning but many people wear two pairs of socks (usually a thin pair and a thick pair), this also keeps your feet nice and warm. When choosing socks, go for materials that dry quickly (your boots should be waterproof but your feet will sweat) and don't slip against your feet (try to avoid nylon). Many shops sell hiking socks with padding at the heels and toes, they are usually a combination of wool and synthetic material but can be expensive.
Obviously, you'll want something to keep you dry when it rains. A waterproof jacket comes in very handy for this, but waterproof overtrousers are also a great bonus if it's really coming down. When picking overtrousers, look for the ones with zips at the bottoms as these are easier to slip over your boots and zip up. There are a huge number of waterproof jackets on sale, some ridiculously expensive and some quite cheap. One thing to note about waterproofs is that as well as keeping the rain out, they tend to keep your sweat in. Breathable fabrics let vapour through from inside but still keeps the rain out; basically, the more expensive jackets are the more breathable ones, but none are completely breathable. A good compromise is a simple jacket with a mesh lining to keep the sticky, sweaty waterproof fabric away from your skin. The clothes under your waterproofs also affect their breathability, more on this in 'Layers of Clothing'
It is possible for a gorgeous sunny day in the hills to turn into a cold and windy one within hours, so you should always carry some extra layers in your pack. It's good to have several different layers instead of one thick one so that you can change to suit the conditions. For example, you'll be a lot warmer going uphill than sitting still eating lunch.
For your layers to work with your waterproofs, you'll need to be wearing things that don't soak up too much water. There are materials available that transport moisture quickly to the outer layers, this is called wicking, cotton is bad at it apparently, but fleecy tops seem to be OK. You can buy special sports clothes that wick moisture away but it depends how much you want to spend.
In cold weather, like from October to April, it's a good idea to bring gloves and a hat with you, and maybe a scarf to stop snowballs going down your neck. The wind can really chill your hands, even in sunny conditions, and a hat keeps in plenty of warmth.
Just to emphasize that you really shouldn't wear jeans. It can start raining without much warning in the hills and it's easy to get caught out without your waterproofs on. Jeans soak up a lot of water and get very cold and heavy when wet, and you won't like it. Tracksuit bottoms, cotton trousers or combats are fine since they dry quickly, shorts also dry quickly if you can stand the cold.