Venus and Mars conjunct on the cusp of the first house. What does this mean?

Attractive in physical appearance.
Venus in the 1st house gives beauty, and often a fine singing voice as well.
Venus conjunct mars gives magnetic appeal to the opposite sex (men find you irresistible).

You are in the correct section as this is astrology, not astronomy. We astrologists understand that you are referring to your natal chart when you say the 1st house.

I think someone who is well educated in astronomy would know, but that's not me. Perhaps if you placed this question in another category. ?

attractive, and sexual

It would have been nice to see what is the sign.......

Mars on the cusp or rising always gives a scar on face or forehead. You can express yourself directly and can be rather forceful when expressing your own point of view . You are not the type to sit quietly in a corner, you need to let everybody know that you are around.
But because of Venus, You will find that, with little persuasion, you can usually get what you want from other people, since you give out the impression that you are willing to give as much as you receive. This may be true, but it may equally be true that you use this ability to get what you want and pursue your own goals regardless of the needs of others. It gives you an iron fist but in a velvet glove.

The conj. of these 2 planets makes you lively and demonstrative. Sensuality can be quite pronounced with this aspect and often find people staring at you. This combo brings much charisma and magnetism. You won't mind the extra attention that this cnojunction brings since Mars loves attention and probably make noise just to attract it.

http://www.psychic.com.au/history-of-hor...

The answer to your question depends on an expert reading of your whole chart. Planets can conjunct your ascendant ( first house cusp) from different signs and houses ( first house or 12th house).

Without knowing which signs and houses, and relating those facts to the rest of the chart, there is no reliable astrological answer to your question. Anyone who attempts an answer is also unreliable and should be ignored.