Does one have to pray to Ganesha before Shiva?

"Does one have to pray to Ganesha before Shiva? Can’t I pray to Shiva directly or will it not have any effect?"

Of course you can address Shiva Mahadeva directly.
In Hinduism however we prefer to “meditate” (dhyana, stuti, etc) rather than “pray” (prarthana). Praying means addressing someone to ASK for something, while the Hindu-Vedic tradition prefers to SERVE through the bhakti process of sravanam, kirtanam, smaranam, pada sevanam, archanam, vandanam, dasyam, sakhyam, atma nivedanam. In this way we establish a closer and more uplifting relationship with the Divine, instead of abasing ourselves as miserable beggars and flatterers. This is the actual EFFECT we are seeking when approaching the Divine.
Meditating on, or addressing Ganesha before addressing Shiva Mahadeva has the purpose of obtaining more inspiration and the proper attitude in approaching God. Ganesha is the personification of progress and development, who destroys obstacles and helps success. And if you have some solid knowledge about the lila of the Personalities of God, it will help you remember how Ganesha was recognized as the perfect worshiper, to be offered respect before starting any activity - how he showed his deep realization of the true tattva of his parents Shiva and Parvati. - Parama Devi