If you do not have any experience, the best way to get started is to get a job on a land-based oil rig. You can get hired as a roustabout, basically a general laborer, work hard for 6 months and climb up the ladder to the position of roughneck. As a roughneck, you will still do a lot of manual labor, but you may also have a chance to supervise a few roustabouts and help out the more specialized positions like the derrickhands and drillers. Show the right attitude, work hard and you may find yourself promoted all the way up to driller (2nd highest rank on the oil rig) in 5 years.
A university degree is not very useful on board an oil rig. On the other hand, a useful technical certificate or diploma, for example as an electrician or mechanic, could land you a job in the motor room or electrical department. These are not the only useful trade skills on board an offshore oil rig. The rig crew need to eat, so a good cook is always welcome on board. A medic also plays a valuable part on the rig. Despite modern technology, an oil rig job is still a pretty hazardous place to work in. Accidents happen, and a medic always has work to do.
An important point to keep in mind is that an offshore oil rig is covered under maritime law. Hence, it is good if you take the trouble to get a basic working knowledge of how it will affect you. Besides this, there are vaccinations and skills certifications specific to where the oil rig is located. For example, in the UK you need to take an offshore survival and firefighting course. Different Canadian provinces have their own First Aid certifications. And different US states have their own requirements.As you can see, there is no secret to getting started in offshore oil rig jobs. Just start small, on shore, from the bottom. Once you prove yourself, you can move to the more lucrative offshore drilling rigs.