I believe the Guild Expedition is the best way to learn fighting.
It gradually gets more difficult. You'll be able to see how far you can go with your current setup. In the first difficulty, you only fight troops an age below yours. Depending on the age you're in, your opponents may also have a certain amount of attack and defense boost. Anyway, a big advantage over PVP is that you can see which units you're up against and choose yours accordingly (for example, if the opponent has an army full of artillery units, you send out your fast units).
You never said if you have any military Great Buildings, like the Statue of Zeus or Cathedral of Aachen, but if you want to fight, I definitely suggest you get a decent attack/defense boost. Because the GE gets increasingly more difficult with two wave battles and enemy units having a defense/attack boost and you will encounter neighbors with these bonuses as well, you'll only be able to get so far without military GBs.
Another piece of advice I have (and again, this is just my opinion. Certain people don't believe in defending their city) is: if you start PVP, i.e. attacking your neighbors, get a defensive GB and some special buildings that provide defense, like Watchfires and Ritual Flames.
You can sometimes get a Watchfire or Tiger's Den in daily challenges. They give your defending army some defense bonus, while defensive GBs give your army both a defense and an attack boost.
Lastly, as other people before me have mentioned, Rogues are an amazing unit if used properly. You can get them from Daily Challenges, events and the GE. You can also win a Rogue Hideout (a training camp for Rogues) from a certain DC or the Challenger's Chest which you can open after you've completed 7 daily challenges.
The best way for me to use them is to fight with 7 Rogues and one regular unit. I move that unit to the farthest upper left corner, away from the reach of enemy units. I then advance with Rogues. They ignore the first hit and transform into the regular unit I keep behind. Note that you can only train two Rogues in one Rogue Hideout without spending diamonds to unlock other spots, so until you get the Alcatraz a GB that gives you military units, this tactic won't be possible.
For now, just try to get as many Rogue Hideouts as possible, build and level military GBs, use an attacker boost in your tavern if need be and practice fighting in the GE. Experiment with different units, learn about their advantages and disadvantages and most importantly, have fun.