No letter is required if you have a Ghanaian or ECOWAS passport. All other nationalities requirea visa. It will be best you call the embassy to establish this if u are not a member of the above categories.
You may enter on an issued tourists (visitors) visa
It depends on your nationality. If you are from a visa free country you don't need an invite, but if yu are not it has requirements. You can either apply for a visa here or apply on arrival. To be on a safer side it's best you apply from your country of origin
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