The choice of solvent system will depend on several factors, including: the mode of chromatography used; the detector type used (e.g. DAD, ELSD, CAD, Fluor, MS); the solubility of the compounds and so on.

Spend some time researching the sample properties and methods on the web first. A key word search will reveal hundreds of useful papers and articles to assist you and will only take seconds of time for you to find. Asking good questions and developing good independent research skills facilitate productive learning.