Riding a giraffe is possible but not very feasible. Shandor Larenty of South Africa has been raising an orphaned giraffe named Mara since 2012 and has been training her for riding since she was 3 months old. Larenty's great-uncle reportedly rode a giraffe as a circus performer in the 1950s.
Giraffes are big and strong enough to carry humans but are poorly suited for riding because of their steeply sloping backs. They can be trained through a process of positive reinforcement with food as a reward. Though they are expensive to keep, they were already used as pets by the ancient Egyptians.