Try the American Embassy. They often have vacancies that they advertise locally. They are advertised on their website. You will need a work visa though before you apply.
I Got Job offer in Ghana,They are offering me with a nice salary, additional benefits etc. Every thing is nice, I checked their website tidalenergyghana.com.So, please find out whether this a genuine company or not? Its a big company or not?
Honestly, it's very difficult getting a "good" job in Ghana once you are actually here. If you're looking for anything official/UN/government/NGO it's much easier to find the job outside Ghana and be transferred here. It is possible to get a job in Ghana in some kind of telemarketing role, or office admin, but you're also competing with a lot of unemployed but qualified Ghanaians for this too. Do your homework on this one as it's not easy.
If you arrive in Ghana on a visitors visa( 2 months) and you are lucky to find a job there, will the Ghana immigration accept your papers for work permit?