To make the biggest possible profit when attacking, send in as few troops as needed to win a battle. Consider the defenses of your enemy and search for weaknesses to take advantage of.
Defense
The most important building in your base is the Boss Mansion. When destroyed, your base will be taken over. That building should be protected the best. Keep it in a central location in a position that is difficult to reach. Position a number of decoy buildings around it to waste the time of invaders. Set your other defenses and road blocks to their best advantage. For example, only have one road leading to the Boss Mansion and lay it out so it becomes long and difficult. It must be surrounded on all sides by buildings to prevent a Dozer from creating a new road to the mansion. Unlock new areas in order to make a square. Place your Boss Mansion in the middle and close off the routes that your enemy could use to deploy troops. Fill all roads possible to create the longest possible road to your mansion. Fill that road with traps and add buildings on both sides so that enemy troops are slowed for as long as possible. Place towers along the way to take out attackers. If you get overrun, watch the replays to reveal your weaknesses and improve them.
Upgrades
Begin by upgrading your storage, then focus on your resource producing units. At first, your storage cannot hold too much resources. Upgrade it a few levels to be able to keep all the resources that are looted from from the single player missions. You will have a few days of protection when first starting, unless you attack another player's base. Use this period of immunity to build your base and complete the single player missions to earn resources. Only attack other bases after the protection period ends. Consider targeting players that decided to end their protection time too early and are left prone. After building a decent base and upgrading your storage for resources, concentrate on upgrading your offense and defense. Upgrade your towers and traps. Also unlock as many troops as possible. Consider reaching the Thug quickly, because it targets defensive towers. Also upgrade your Bars to increase the size of your army.
Workers
You only have one worker to start with. Hire a second one as soon as possible to build your base faster, then begin saving for a third worker.