First get a dust blower. Then hold your camera such that when you use the dust blower or pressurized air can the dirt falls out of the lens mount. in other words hold the camera in one hand slightly above your head in front of you. So when you blow air into it what ever loose dirt/sand falls out of the lens mount opening. If you're not sure send it in to the camera service centre. These things are delicate and if you damage them (mirror and/or sensor) you'll have to replace it. Ask the service center tech to show you how to clean it safely.