What is it dissolved in? You would need to dilute the solution unless you have access to automatic pipettes with which you could reliably measure 10 microlitres or less. Otherwise, your best bet is diluting the solution to 1000 ml - that way you will have 1 mg in 1 ml, and you can measure out 0.25 ml using a 1 ml syringe for example.
E: is the whole bottle 15 ml, and it contains 1 g of fblam? Basically you want the concentration to be 1 mg per 1 ml, so if you have a total of 15 ml of the solution, you will need to dilute it to 1000 ml (1 g = 1000 mg, 1000 mg per 1000 ml = 1 mg per 1 ml). Likewise, if you have, for example, 25 ml of the solution with the same concentration, you will need to dilute it to approx 1667 ml to get 1 mg/ml concentration.