You'll probably notice more difference with a good pair of cycling shorts than any other cycling garment, so make shorts a priority.
Most road cyclists wear lycra shorts. These are close fitting, with a padded seat (wear them without underwear, but do wash them after every ride!) They also have a longish leg, often with elastic 'leg-grippers' to prevent them from riding up, bunching or chafing. If that's not your thing, there are some more fashion-conscious shorts available. These have a baggier cut, but still with a reinforced seat. Mountain bikers often wear these.
You can also buy special
cycling underwear, or shorts liners. These too have a padded seat and avoid any poorly positioned seams that could cause problems.
A
wicking base layer is always a good idea (See
Basic principles). On top of that, you can wear a
cycling jersey. These are usually acrylic or polyester, with a zip neck (or even a full length zip) for ventilation, and have three pockets at the rear to carry your essentials (like your wallet and an energy bar or two).
Leisure cyclists and mountain bikers often prefer to wear a more casual looking light
microfleece instead of a jersey. That's fine, but these are usually not quite as close cut as road garments, so they're more suitable for situations where speed is less important.
Cycling jacket
As for a
jacket, wearing more thin layers is much better than wearing just one or two thick layers. You can adjust for the temperature much more easily and the potential for greater insulation is much better.
You may well invest in a variety of jackets - a good quality
waterproof, a
windproof, a light
shower resistant jacket, a slightly
thicker jacket for cold days and perhaps a sleeveless
gilet for those in between conditions.
You'll also want a pair of longs or
tights for cooler days. Cycling specific tights are better than regular track suit bottoms because they have a longer back and are a closer fir than most other sports garments - so they won't get caught up in the chain or flap about in the wind.
Keen cyclists will probably add some
arm-warmers and
knee-warmers. These are ideal for cool days or for warming up, as they can easily be removed when conditions warm up.
You can also buy
socks specifically for cycling. These are cut very low (anything longer and socks just get heavy when wet). Coolmax is the perfect fabric.
Of course, if you start riding specialist events like downhill races, or time trials, then there's a whole new uniform to consider...