Your equal priority, along with upgrading your walls, should be upgrading your turrets too, because the stronger your defenses are (and the more of them you have), the quicker that enemy troops will die when they try to attack you. In fact, strong defenses are by far the fastest way to earn medals, because when other players attack you and fail to earn any stars whatsoever, you gain medals rather than losing them.
When you attack people, take a good look at their credit markets and their alloy collectors (they will appear visibly full if they are full), as well as their total number of available resources. If the last digit is either an 0 or a 5, they are likely to have their credits and alloys stored in the markets rather than the depots and the vaults. Typically other players will place vaults and depots under heavier fortification than the markets and the refineries are)
Join a squad as quickly as you can so that you can request troops and so that you can send troops back. If you are having trouble finding a good squad to join, go to the comments page here, or go to the Facebook fan page for this game or the App Store review page (or just ask your friends or coworkers or something), or start your own clan and start inviting people.
For a good idea on how to organize your defenses and walls without doing too much thinking, simply battle against a ton of players, and take note of the ones who frustrate you the most, who it is the most difficult to win against or who it’s the most difficult to earn any stars off of. Then copy what they do for their bases, and do it for your base. This will make it quite easy to improve quickly and excel at defense.
If you are the type of player who decides to let their shields run their course, rather than cancelling them out in order to attack other players, then max out your alloy refineries and credit markets in level, and build as many of them as you have unlocked. Then you can let them sit for as long as you want (remember, stick the headquarters outside of the base for free and easy shields), then collect from them every so often until you’re as loaded as you want to be.
Build the shield generator as soon as you can and upgrade it as high as possible. Also, to get rid of shield generators, you will need specific heroes, so don’t attack anyone with a shield without those specific heroes. Stick your most important buildings inside of the shields, and guard them HEAVILY to prevent heroes from attacking them.