The One City Challenge is, as the name suggests, a challenge to win a game of Freeciv with just one city. The basic rule is that you may only build one city. Some variants allow other cities to be captured, but if they are, you must build settlers to get them down to population 1 and then disband them as fast as possible. The more normal approach is not to allow capture of any other cities.
You'll also need to make sure the space race is turned on, as winning by any other method is very hard or impossible.
Tips and tricks: Edit
Early game Edit
Your biggest need is going to be research. Try to build on, or at least near, research producing specials such as silk, wine, gems, spice, gold or whales. (Okay, you can't actually build on whales. And I probably would only build my city on silk or wine).
Because you're so short of research points, the order of your early research is crucial. My suggestion is Pottery first (since both Granary and Workers will get your city growing more quickly); tthen Bronze Working (to build the Colossus for an early trade boost); then Writing (for Library). You may then need Ceremonial Burial if your city is already at size 4, but that's on the path to Monarchy anyway.And after Monarchy, you priority must be Literacy, and then it's a good idea to go for Republic.
The only reason i would vary this order was if I had a noisy and potentially aggressive neighbour, in which case I'd throw in Warrior Code, probably after Writing.
Don't build things you don't need, like Barracks; if you have nothing useful on your build list, go for Coinage.
Mid game (up to Industrialization) Edit
There are five Wonders of the World that are crucial to success in this challenge. Four of them are the Wonders that affect one city only: the Colossus for trade; Copernicus' Observatory for science; King Richard's Crusade for production; and Shakespeare's Theatre for happiness. The latter is the most useful wonder in the whole game; it makes everyone in the city content, all the time. Once you have built this, you can sell your Temple. And you never need to build Cathedral, Colosseum, Police Station, Hanging Gardens, Michelangelo's Chapel, JS Bach's Cathedral, Women's Suffrage, or Cure for Cancer. That's a HUGE saving in production.
The fifth Wonder that is virtually essential is the Great Library, hence the emphasis on researching Literacy. You'll probably be in a race to build this with other Civs. Use your collected Coinage to buy the LIbrary as soon as you possibly can. You may even need to delay switching from Despotism to Monarchy to avoid the few turns of research/production downtime.
The only other mid-game Wonder that you'll want is Isaac Newton's College, for the research boost. Adam Smiths' College will only save you 2-3 gold per turn, which is hardly worth the cost of building it. Leonardo's Workshop will save you 81 gold per upgrade, but you shouldn't have more than 4 or 5 units in total (unless you have a really aggressive neighbour), in which case your total upgrade cost will be 300-500 gold.
Once your government is a Republic, you may want to trigger a President's Day Sale to get your population up to 8,then 12, and maybe beyond.
Key research goals after Republic are: Construction; Engineering; Astronomy; Medicine; Theory of Gravity; and Sanitation. Once you have done these, you'll be feeling the need of higher production, so it's time to go to Railroad and then Industrialization.
Late Game Edit
I hate to say it, but the game can become rather dull at this point, and (if you have lots of other civs in play) slow as well. You'l want Refrigeration so your city can grow to 25-28 people (those who can't work the land should become researchers). Refining will let you build a power plant, or Electronics a hydro plant. Then it's a long slog to Minituarization and then Computers, for Offshore Platform, Research Lab and SETI Program. After that it's time to go to Space Flight, possibly via Robotics for Manufacturing Plant.
If you start having trouble with your neighbours, consider building the Eiffel Tower or the United Nations.
Remember to boost your defences before starting your spaceship, as everyone will declare war on you when you do, and losing your city at this point would be a bit of a disaster.