First you have to know that Arabic language has many dialects, But the majority of Arabs understand Modern Standard Arabic, So if you are new to Arabic language it is better for you to speak modern standard Arabic, So Arabs with different dialects could understand you. The answer for your question is "حظ سعيد" -haz sa'eed-, this is the literal translation and it meas you wish him/her good luck, but most of native Arabic speakers would also say “بالتوفيق” -be al tawfek- and this means you wish him/her reach success.