If you are stimulant-naive (don't do other stimulants like Adderall, Ritalin, other amphetamines, MDMA, mephedrone etc) - try maybe 5mg. If that does nothing try 10-15mg.
Focalin is the active isomer of methylphenidate; it's basically 2x as strong as racemic methylphenidate found in Ritalin. 10-30mg are good doses for that, so try 5-15mg of Focalin. Make sure to eat and drink something so you have some energy. The "crash" comes after 3 or 4 hours.