What am I to do with my "subordinates"?

I am beginning to work as project manager in a small firm. It is so small that I "manage" everyone except the very main director, I give them tasks and terms, I monitor their progress and encourage and fine them in accordance with their successes.

This all is fine, but it seems that my style of communication is going to cause problems. You see, I almost always seem to be cheerful and playful and carefree. I smile and I joke, that's just how I talk.

So the question is: will this style of mine make my guys think that I'm really not serious, and my rules are not to be followed, and my words are not to be taken seriously?

If so, what do you think would be better and why: to wait a little until they realize that behind my smile I'm always quite serious, or to make up 2 regimes of communication - "working" and "not working", or to be always serious and severe? This last case has its drawbacks - I don't want to end up being everyone's enemy (literally everyone!), because people never work well when they work for enemy, and I do want them to work well.

Please don't tell me that I should decide it myself - I certainly will; I just want to hear what you can recommend :)
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I think it is a great style. I wish you were my boss. I would evaluate their personalities. If they are serious and cut throat, by all means be business. If not and they joke, I would joke. You can have a balance and happy medium.
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I don't see why you cannot be friendly and professional at the same time. To make sure that there is no misunderstanding, when you hand out assignment don't joke around. You can be serious without being mean.