Welcome to CFC, flymi!
One of the best ways to get advice on your game is to post a save. That way, other players can download it and really take a look at what's going on with your economy. Also, what version are you playing?
Delphi456 has already hit on one of the most common reasons for lack of gold for new players: Lack of roads. In addition to hooking up resources & luxes and the movement issue, the commerce bonus of Republic adds one commerce to every tile already producing at least one commerce. Zero-commerce tiles (like the forest S-SW of London) remain zero-commerce tiles. If you add a road to it, though, it becomes a two-commerce tile. The easy way to ensure that you've got enough roads is simple: Do not to let a worker leave a tile without building a road.
The lack of roads is often caused by a second common issue: lack of workers. Shoot for about 2 workers per city. You can get away with fewer if you're playing and industrious civ, but that's a good rule of thumb. Also, don't overlook the value of slaves. They cost no upkeep, so don't pass up an opportunity to buy them from other civs. Killing other civs' settlers & razing their cities also gives you slaves. They work half as fast as normal workers, but (again) they're free in terms of upkeep.
Looking at your screenshot: First of all, what's your victory condition? I'm going to guess that you've built lots of unnecessary improvements. What is "unnecessary" is partly defined by your victory condition. I see barracks, temple, library, granary, courthouse and The Great Library. Not all of those are necessary. A courthouse reduces corruption. London is your capital. There is no corruption in your capital. So the courthouse is costing you gold, but not doing you any good. If you're shooting for a cultural victory, the temple's a good idea. If you're shooting for conquest, you don't need it.
I also see that London is producing 19 culture per turn. What else is hiding down there that you've built? Also, I'll suggest a trip to the War Academy. Some article to look at are: Cracker's Opening Plays, and The Four Rules of Wonder Addiction.
Your luxury slider is unnecessarily high. I see 10 happy people and 2 that are either content or unhappy. To keep your city out of disorder, you only need at least as many happy as unhappy people. For example, a size 10 city:
->6 happy, 4 unhappy = no riot;
->5 happy, 1 content, 4 unhappy = no riot;
->4 happy, 2 content, 4 unhappy = no riot;
-> 4 happy, 1 content, 5 unhappy = riot.
So you can turn your lux slider down a notch or two.
While I'm on the subject of happiness, can you build a market yet? They increase tax revenue and add to happiness, so London could use one.
Finally, I see 4 spears and a warrior guarding London. A republic doesn't get any MP effect from military units, but I see stacks in the other cities. So I'm going to guess that you've put several defensive units in every city. If that's the case, you're spending gold on units that aren't doing you any good.