For the Schengen countries you do not need an invitation, but you need to show you have enough money and a good reason to return home, and Norway, Greece and Switzerland are all Schengen countries.
The money is not a given amount but showing that you get in enough money more than you need for your life at home that you can afford to spend it on travel.
Good reasons to return home are dependents (elderly parents, wife, children) a good job or own company, owning a house, being in the later stages of a study you can and want to complete.
You do not need all of those but enough that the visa officials are convinced it is very likely for you to go home rather than stay in the country you visit.
The amount of money is also based on the travel cost (on average) from your country to where you want to go and the cost of staying in that country.
The different countries mention the minimum amounts needed for staying there. For Greece it is €50 per day (minimum €300 for a stay of up to 5 days); for Norway there is no fixed minimum but their guide is NOK500 per day for visitors who are not staying with relations or friends. For Switzerland it is approx CHF100 daily, (thanks to @Traveller, from a comment.)
There are people who try to use all their life savings on one travel. Immigration will find this suspicious and likely deny them a visa.
Being from Nigeria is not going to help you, while still many people from your country get visa, a relatively high percentage of visa applications from people from your country are rejected.
Having a very strong application is needed, but an invitation letter is not often part of that.