+1

I ride 10 miles each way. If I ride in jeans and cotton briefs, I get nasty chafing in places you never want to chafe. So I wear bike shorts. I also sweat a lot when I'm riding, even in cold weather, so I wear quick-dry, non-cotton shirts. For commuting I prefer loose fitting, usually non-cycling-spefic shirts. For other cycling activities I have tight-fitting cycling jerseys. Both shorts and shirts are also better than my "normal" clothes for riding in the rain. I wear MTB shoes with cleats so I can clip into my pedals and not worry about my feet slipping off.

As a first cycling-specific purchase, regardless of the length of your typical ride, I would definitely go with a low-invisibility yellow cycling jacket. On a clear day, these things are visible from two miles away and they almost always indicate there's a cyclist on the road. Even at night a jacket like this is more visible than most other clothing. A bright read shirt, for instance, looks fairly black at night, but a day-glow yellow jacket looks yellow. Most cycling jackets also have reflective details that light up in a car's headlights.