Transport won't pose any problems during your trip to Armenia. It's easy to get around and you'll find the mode of transport that suits you best.
During your trip to Armenia you'll find it easy to tour towns by foot, Erevan being the only exception. But that won't pose a problem, thanks to its numerous bus lines. Make sure you tell the driver where you're heading, so that you don't hop on the wrong bus. Travelling by bus is by the far the cheapest option.
The second option is a bit more expensive but much quicker - taking a minibus or minivan, AKA marshrutnis. Once again, confirm your destination, so that you don't find yourself being dropped off on the wrong side of town.
Travelling by taxi is the most practical way of getting around, but it's also the dearest. Be warned that drivers may not turn the meter on, so agree on a price before you get in.
For the same price as a bus ticket, you can use the capital's single underground line.
If you want to travel between towns without spending a penny, it's worth noting that hitchhiking works well in Armenia. If not you can always catch a bus or marshrutni. They're popular forms of transport and a great way to meet the locals.
If it's speed and comfort you're after, some taxi drivers will agree to drive long distances.
Trains are indeterminably slow. You'll need to have all the time in the world and be a real train lover if you're going to squeeze into a wagon compartment.
As always, the best way to travel at your own pace and where you please is to hire a vehicle. Be careful when you're driving along little roads and make sure your sense of direction is fully tuned in, as road signs are rare.