I play the germans a good bit (and I only play against the AI). I can win on Tough all the time and on tougher some of the time (depending mostly on map and opponent).
Here are some tips for learning how to beat the AI (as germans or any other nation really):
1) Play the game set on Tough. Just play for 10 minutes or so...do your best to stay in the game. Resign. Watch the replay and switch the view to the AI's point of view (or just lift fog of war). Watch what the AI builds and researches) and how quickly it gets its second city up. Learn how to emulate this. If you are trying to win by booming your economy, then it is critical to get that second city up and commerce researched ASAP.
2) Check out this link: http://www.bighugegames.com/riseofnations/strategy_booming.htmlIt is BHG's guide to "booming" your economy and is really a a good basic strategy...as you learn the game, you can adjust this strat to suit your play style and tastes.
3) To defend against rushes from the AI, research Military Science 1 shortly after getting you second city up (probably just before or just after hitting classical age). Build 3 or 4 units, at least two of which are archers/bowmen (three is better, but that's expensive). The computer usually does a Heavy Infantry rush and archers are the best counter to that. When the rush comes, engage with your groud troops (build a few more if you can afford it). Send two citizens to garrison in a city if it is under attack or near the enemy army (just two..3 if you are playing chinese) This will cause the city to fire on the enemy...you need 6 citizens (7 if chinese) to get a bigger boost from the city and that will cripple your economy for too long).
4) The German's greatest economic advantage is that they can get Granaries and Lumber mills early and cheap. Use that. Once you've built 4 farms and have at least 4 or 5 wood cutters at your first city, build a granary and a lumber mill, then max out your farms and wood cutters at that city (add more wood cutter camps if you can). Once you hit the classical age, do the same thing with metal mining...build a smelter as soon as you can. You also can research resource collection upgrades at the granary, lumber mill and smelter. Always do that as soon as you can. This will keep your resourece collecion at or near max. If you start running up against your resource collection cap (you see flashing yellow indicators in the upper left) research commerce. Basically: Research Science. Research resource gathering at granary, lumber mill, etc. If you see flashing yellow, research commerce. If no flashing yellow, build more farms, woodcutters, miners and granaries, lumber mills and smelters (probably around 10 of each if you're the Germans...a few more if not). Repeat until game over.
One of the big economic weaknesses of the germans is that while it is easy to max out food, wood and metal collection, it is difficult to have your wealth keep pace (and wealth is important for gathering knowledge and building archers). If you have too much food, metal or wood and not enough wealth to do what you want, sell stuff at the market to convert it to wealth. Build a market at _EVERY_ city (+10 wealth per market...that's important). Max your your trade routes by building caravans (press F12 twice to see how many trade routes you have and how many you can have). And research taxation upgrades at your temples.
5) Militarily, use control groups (I have 1 = my main army, 2 = my secondary army/cavalry, 3 = my fleet, 4 = a group of 8-10 fighters, 5 = a group of 4-5 bombers, 6 = misc, 7 = all my ports, 8 = all my seige factories, 9 = all my stables, 0 = all my barracks). Also build a castle at the city where the AI attacks (just behind it) and garrison 3 or so archers in it). Build a general and use his "decoy" ability often. Use your decoys as a screen for your real troops. Include a scout or 3 in your army to spot enemy decoys and spies. Build at least one tower and research attrition upgrades. When the AI is on the attack, use cavalry to take out enemy supply wagons. The Jerries get two free fighters at airbases. As soon as you hit the industrial age build an airbase (especially if your opponent hasn't reached it yet) and use those free aircraft to harrass your enemy.
Since you're playing the AI, don't be afraid to use PAUSE while you're learning. If you plan to play online against human opponents, ween yourself off PAUSE because you can't pause in those kind of games.