M. Jalal Uddin, this depends a little on the formulation. If possible, check the product insert. Sometimes you only need to add a defined amount of pure water, as the salts already are included in the lyophilized prepraration.

Otherwise, I'd try PBS or PBST (depending on your assay) with a little (0.01% m/v) sodium azide as preservative.

For dilute solutions (below ~100ug/ml), include (biotin free) BSA or another inert protein to prevent your conjugate from sticking to surfaces.