Nigeria is a country in Africa. It is bordered by Benin to the west, Niger to the north, Chad (via Lake Chad) to the extreme east, and Cameroon to the south. With a population of 167 million, approximately one in five Africans is Nigerian. This also means that one in five African criminals is Nigerian, a fact that has given them a fairly bad reputation for high crime and low safety in their country. The fact that Nigeria is often in the world news for negative reasons - whether the Biafra conflict, the struggle for oil in the Niger Delta (and the routine kidnapping of foreigners that resulted), government corruption, the current Boko Haram terrorist threat in Kano and other northern areas, or the worldwide Nigerian email scams - has extended the country's bad reputation worldwide. However if you are aware of the risks and a vigilant traveller, it is possible to have a wonderfully rewarding experience hitchhiking Nigeria.
In Nigeria, as is common throughout Africa, the long distance public travel system is a choice between big buses, or smaller vans or cars. Some collect passengers only at the bus stations in towns, but many will stop wherever there are passengers waiting for a ride. Thus unless you are willing to pay for a ride you must learn to distinguish these vehicles from personal ones, and not flag them down. Personal vehicles may also ask for money (sometimes more than at the station).
It is best to try to find rides once outside the cities. Inside there are far too many taxis/moto-taxis/vans/buses hungrily searching for passengers. You can easily recognize them by typical green color with yellow strip in the middle. Not to mention hitching in plain view of hard working locals may instigate negative feelings from those looking desperately to make their $1 profit for the day. Get to the end of town and signal vehicles by holding out your arm horizontally, palm face down, and waving up and down.
That said, Nigeria is a very friendly country and many people will go far out of their way to help you. They are proud citizens and weary of the negative view that the rest of Africa and the world has of their country, so they will likely go out of their way to show you wonderful hospitality and help you experience their vibrant culture.
Finding a place to pass the night is also as easy as asking to speak with a village chief or community leader. All the better if you have a tent. Even national parks (i.e. Yankari) or other tourist establishments (although these are admittedly very limited) may take you in if you approach in the right way. In the cities Couchsurfing and the like are becoming popular.
It is important to know which areas are safe and which are risky, as this can change rapidly. Locals will usually give reliable advice and contrary to popular belief, Nigerian police can be very helpful. As in many parts of Africa they will find it strange that a foreigner is travelling by hitchiking (most locals would not do it as they see it as begging and therefore below them, particularly on main roads where public transit is readily available). With all the political coups that reign throughout Nigeria's 50 years of independence, the police have learned to be somewhat suspicious and they may question your motives, so give them no reason to suspect you of being a spy. They are inherently suspicious of cameras, so keep them hidden around police and do not take pictures of government buildings. Otherwise police will generally not hinder you or ask for bribes. As is the case in most African countries, requesting bribes is reserved for rich people in shiny SUVs flashing fancy cameras and iPhones, not for slightly grubby looking foreigners 'begging' for free rides. In some areas, particularly farther north and east, the police may prove happy to assist with helping you find rides, or even offering a place to sleep.
If you are riding in a car altogether with locals during the night, you can be asked by the police if everything is OK. This is mostly because they want to make sure you are not kidnapped. Don't be surprised and/or blindsided.
It is also important to note that there is Islamic Sharia law in effect in the northern states. This issue has become more serious in recent years due to the increased activity of Al Qaeda affiliated group Boko Haram. Again, make sure to look into the current situation in the area you plan to travel through, and always ask for advice. Despite the Sharia law and its tendency to restrict women, it is possible for a solo woman to hitch solo throughout the country without issue (as Alyssa did in 2010), however being highly assertive is important.
Lastly, watch out for bad drivers. You will see a lot of wrecks on the side of the road. It is all the time handy to have some "first aid kit", for instance an old one from a car, with gloves.
English is the official language, and thanks to British colonialism and the structure of the country there is a relatively high level of education among the general population - most people speak at least conversational English. Most Nollywood films are in English or its relative creole/pidgin (which is almost an entirely unique language with its own grammar structure - difficult, but possible, to understand for the untrained English-speaker's ear). There are also over 500 local languages spoken throughout the country, some regional dialects, some completely unique. Every Nigerian will likely speak one or more of the three national languages: Hausa (north), Yoruba (southwest) or Igbo (south east). It can be helpful and a sign of respect to learn greetings and basic phrases in the language of the region you are traveling through. Even giving a shot at 'hello' will get smiles from anyone listening.
Towns and Cities
Mighty Lagos with its population of 8 million and all the congestion that goes along with any third-world megopolis. This city is huge and sprawls onwards for many kilometers. Getting from one end to the other can take many hours at any time of day. To hitch out the most practical way would be taking a cheap combi out of the main city to somewhere less populated and wait on the road there. Traffic can be backed to a crawl for hundreds of kilometers surrounding Lagos if construction is being done.
Nigeria's capital Abuja is a huge, modern expat city that was planned from the ground up, and it shows. Building are new, roads are nicely paved, there are parks and open spaces, and walking around the city is a pain because everything is so far apart. This can be a benefit to hitchers because as long as you are away from the main downtown core it main be possible to get a lift out though taking a combi to the outskirts and waiting there would still be faster.
All borders with Niger should be quite easy to cross, although the further east one goes the less vehicles one will see. An interesting idea is getting a ride with the used car smugglers who pick up cars in the Benin port and drive them north to Niger and then smuggle them across the unmanned desert borders (apparently the thousands of kms of extra gas and import fees for both Benin and Niger are still cheaper than dealing with corruption at Nigeria's ports).
Either from a boat leaving from Calabar, through the jungle where during the rainy season mud will be piled as high as the car on either side of the road, or the road further northeast. All are feasible. The jungle is fun.
Likely the shortest route for many overland travellers, but like the car smugglers, corruption and shear volumes of people may create hassles in the south. Seldom used border crossings further north would likely avoid this.