Though I like to think that most ASW readers are careful with their dress generally, the first time a man takes a prospective partner out socially sets the tone for any subsequent relationship. That means some extra thought should be given to the occasion, and to dress (you knew we would be getting to that did you not?).
Appropriate clothing is always situational, which is the reason Richard Gere sent Julia Roberts out to buy clothes before being seen with her publicly in the film Pretty Woman. Sending one's date shopping in advance of the occasion is, of course, unlikely to be appropriate however desireable it might be theoretically. That leaves us to ask ourselves what will make the datee comfortable. After all, their comfort will also be ours. Tune the formality level to the other person.
That said, the first date is safer when the number of conversational opportunities are relatively controllable. A concert, for example, or a film, provides space to determine if the other has anything to say without creating the possibility of long mutual silences, as a one-on-one dinner might when there proves to be less personality on the other side of the table than one had hoped. After all, the evening is usually easily extended when things are going well. The opposite is harder. But I digress.
The types of occasions that make for successful first dates leave us with a surprisingly narrow range of clothing choices. Need I say that black tie is unlikely? A suit will generally set too formal a tone, most odd jackets are wrong after six, and men who would choose to go jacketless are surely reading some other advice. This leaves us with the blazer or a velvet jacket by default, and I like the velvet best if one is available. In dark blue. Combine it with polished black slip-on shoes or velvet slippers, and relate the trousers to the occasion. Denim works well for films if you own any denim, with gray flannel a better choice for the theater. Subtract socks when the weather allows and add a bow tie if your self-confidence permits.
Good luck to you.