There are many reasons your gums could bleed, but in any case you should see a dentist to determine the cause and treatment options as gum condition is closely tied to overall health.
In some cases, if you experience bleeding from your gums when you brush, it may be a sign of gingivitis, the early stage of more serious periodontal disease. Gingivitis is reversible and preventable if effectively treated in consultation with your dentist. You may also experience bleeding if you begin a regimen of flossing, which should dissipate in a week or two.