When attacking someone, you gain between 5% and 10% of their land, money, power and civilians. But that is only if your attack is successful.
To not fail shouldn’t be very hard if you calculate everything correctly.

There are several different bonuses and stats of a kingdom affecting their offense and defense, and knowing all those are important if you want to be successful when attacking. Even if you can use the attack calculator which can be found in the menu to the left to calculate attacks, you still have to know what to set different values on that one to, in order to make correct calculations.

How to calculate the defending kingdom’s defense
Every military unit has a specific amount of offense and defense points, so it’s good if you have a calculator when doing this. The defense points are only used for the military units which that person has at home, so if the defending kingdom’s tanks are all out, you should not count with them.

Things which affect the defense of a kingdom are ("Expect" is what value you should expect this statistic to have in your calculations, if you don't check it before attacking):

Below is an example of how a person’s SoM can look (you can get this by going to the probe room, enter the sector number of the kingdom which you want to probe, pick the kingdom in the list, and use the command "Spy on Military" in the list). SoM is actually the only stats really needed to attack someone:

If you attack someone, you just have to calculate defense for the military in the first row, Home Leader. The rest of the army is away because the kingdom has recently attacked. This persons base defense is 12*1 (Soldiers) + 1570*4 (Laser Troopers) + 700 * 5 (Laser Dragoons) + 2063 * 9 (Tanks) + 200 * 4 (Tactical Fighters), which is a total of 29,159 defense points. The Dragoons and the Troopers are offensive units only, so they don’t count to defense at all. Then multiply this with that kingdom’s bonuses to get his total defense.

Example
I want to attack the kingdom mentioned earlier, with 29,159 defense points. But that kingdom is Oceanic, and is Sector leader. He also has full military research. None of us is at war or peace with the other.

Then his defense will be a lot higher than just 29,159.
100% + 15% (oceanic) + 10% (sector leader) + 30% (military research) = 155%
His total defense will then be 155% of 29,159 defense points = 45,196 defense points.

How to calculate your offense
When you’re done with the defense calculations, it’s time to calculate your offense as well. Your offense must always be higher than both the defending kingdom’s defense and higher than 2* THE DEFENDING KINGDOMS LAND (important!) to be successful. The minimum defense for all kingdoms is always 2 defense points/land, so never send less than that… because then you will fail. This means that you will need 500 offense points to break a 250 land kingdom which has no defense at all (or a total defense of less than 500 defense points, including bonuses), since 250*2 = 500. And 500 offense points is either 125 troopers or 56 tanks, without military bonus. If a kingdom has more defense than land defense, you won't have to care about the land defense in your calculations.

Things which affect your offense (all those things, except for the defending kingdom's shields, can always be seen by looking at your own different stats) are:

Alliance info can either be viewed by clicking on the alliance name at the top of a sector list, or by looking at the attack room for a sector which is in a specific alliance. If the alliance for example is at Peace with you, it says "Alliance relations: Peace" in there. If it doesn't say anything at all, the alliances do not have any specific relation to each others, and attacking is not affected.

For every additional War Leader you send, you get +3% offense, but you don't get any bonus for the first one. So if sending 3 WL's, you get 6% bonus to offense.

If my kingdom then is Sector Leader, Mystical Lands + out of power, and I have 21% military research, send three war leaders when attacking (3% extra offense for the additional two), and the defender has 5% military shields (which decreases offense of attacker) my offensive power will be 100% -6% +10% -20% +21% +6% -5% = 106%, which means that I will have to multiply my offense with 106% (and the defender’s defense with whatever total % you get from his bonuses) before comparing them, to see if I will be successful.

It is usually far easier to calculate attacking with a Star Kingdoms attack calculator (like the one at the bottom of the menu to the left), than in the head, so attacking is actually not as complicated as it might seem above. You won't have to be a mathematical genius to hit someone successfully, all you have to learn is how to find out what the different stats of a kingdom are, or what you should expect them to be, and the calculator will be able to do most of the work for you.