First of all, I am not immigration attorney or anything like that, but i have read a lot about immigration procedures. Yes, you can be sponsored for a visa by an employer. But you have to have a good relationship with your employer or future employer, and ask him/her/them to sponsor you ( it would be even better and easier if they would offer to sponsor you, of course). Also most likely they (the company) would do all your paperwork. Here's the biggest hurdle: Employment visas or work Visas (they're called something like that) for unskilled workers are very, very hard to get; almost impossible while. It would be much easier if you are college educated with a degree on a field in demand of workers in the country you wish to work in. And yes you would be working in that field/career and with that company to get your visa.
I add to this, if you would like to migrate to Canada. If you get a job offer letter from a Canadian employer certified by Arranged Employment Opinion (AEO) that will definitely improves your chances to migrate to Canada under Skilled Worker scheme, if you fall into any of the 38 occupations list like Auditor, IT professional etc.
For Nigerian, If you have an employer that will sponsor you then contact an Nigerian registered travel agent and they can prepare and lodge the application for you and your employer (at a cost). This is often the fastest and most effective way. They can also give you advice on whether you can get an employer sponsored visa for your occupation. A working example of reliable migration agents are DNA Migration.
March 27, 2012, 11:19:20 PM