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It is important to have a sense of what your first date activities will entail before determining what to wear. If you can't narrow it down to specifics, at least try to discern how formal the atmosphere will be. It is generally better to be over vs. under-dressed, so if you have to guess err on that side of matters. One exception, if you have scheduled a day time event things will automatically be more casual.
Particular to women, be careful about dressing sexy. It is natural to want to appear attractive when first meeting someone. However, given the person does not know you very well, a lot of assumptions will be made according to how you present yourself. If you believe you are interested in becoming sexual with this person fairly quickly, there may be no problem with communicating this idea through your choice in dress. If, on the other hand, you want to go slow you may inadvertently contribute to creating a situation you don't want to have to deal with.
I know that many have met and married from meeting online, but my experience has been that people DO pad the truth on who they are etc. I now only use the internet to do just about anything other than meet people. I can do that in person in "real life". We must not hide away typing to people sitting in their housecoats if we can meet people on the outside.