Usually diagnosis of Crohn's is based over your physical symptoms ( bloody stool , diarrhea , nausea , vomiting , weight loss , lower abdominal pain... Symptoms can vary for everyone) , blood tests (inflammation markers,CBC) , ct scan and colonoscopy for biopsies and images. If they found something they will assess the intensity of the inflammation and how much of the colon is inflamed , your current overall health and start to prescribe you medicine accordingly.
Medicines for Crohn's are many and you might want to look into your diet as well as it affects Crohn's greatly. The doctor job would be to induce remission and to lessen your symptoms. There's an undiagnosed subforum in the support section where you will find others in similar situation. Good luck and keep us posted.