Laboratories routinely describe the concentration of chemical solutions in terms of mass chemical per unit volume of solution. There are numerous units that can be used for these concentration values, but a common combination is nanograms (ng) per microliter (uL). Parts per million (ppm) are also often used. This is a unitless measure of concentration that describes the "parts" of a chemical, such as grams, found in one million equivalent parts of solution. You can convert ng/uL to ppm if you know the density of your solution.
Write out your concentration value in units of ng/uL. As an example, consider a solution of concentration 45.8 ng/uL.
Rewrite your concentration value using the same number, but changing the units to milligrams (mg) per litre (L). The reason your numerical value does not change is there are 1,000,000 ng in a mg and also 1,000,000 uL in a litre, so you have not changed the ratio between the upper and lower units. In the example, you would write out 45.8 mg/L.
Divide the concentration of your solution, in mg/L, by the density of the solution, in kilograms per litre (kg/L). This calculation changes the concentration into units of milligrams per kilogram (mg/kg). Since there are 1,000,000 mg in a kg, this value is the concentration of your solution in parts per million (ppm). In the case of the example, if you had a solution of density 1.04kg/L, you would divide this into 45.8 to obtain 44.0 mg/kg, so the solution concentration is 44.0 ppm.
If you have a dilute solution in water near room temperature, which will have a density close to 1g/ml, the concentration in ng/uL will be almost exactly equal to the concentration in ppm.