I am not a necromancer, and am only adding this, as there is there is currently no other strategy here for the necromancer hero class.
If you have anything to add, but can't edit this, then comment in a thread below.

The short version
-Put your skill points in to mana until you have 65, and cast alchemy for extra gold whenever possible. After you have 65 mana, put the points where you want
-Start your land as 75% mines, 25% barracks.
Skill points

Firstly, with any hero, you should start off by putting your hero points in to mana.
The reason for this is the spell "alchemy" which when cast, gives you an additional 85%(average) of your kingdoms gold income.

I'd advise building you mana up to 65 to begin with
Alchemy costs 30% of your mana, or 20 mana (if 30% of your mana is less than 20), so if you increase you mana after 70, it will always cost 30% of you mana to cast, and before 60, you can't store enough mana to cast alchemy 3 times.
Also, mana recharges at a rate of 5 mana per hour before you have 50 mana, and after that, it recharges at 10% of your mana. This means that below 50 mana, alchemy takes 4 hours to charge, and you can't store 3, and after 65, it takes 3 hours to charge, and you can store 3. (at 60 mana, the first takes 4 hours to charge, then the next two take 3)
An advantage of the necromancer, is that they start with more mana than most other heroes, so they can reach 65 mana quicker, and can take more advantage of alchemy at an earlier stage.

There is however, also the issue of the "raise dead" spell, which costs 25 mana, and converts 10% of you army worth of soldier in to zombies. If you expect you'll often have an entire army of soldiers, and will need to convert them all in to zombies immediately, then you'll want to have 250 max mana (for 10 casts of raise dead), but in my opinion, it's not worth if.

After that, spread your hero point however you wish.

Units

The elite unit of necromancers are zombies, which have 3 attack, 3 defence, and an upkeep of 3 gold (per hour), which is the worst stats of all the elites, and is effectively the same (in attack/defence and upkeep) as having 3 soldiers.
It may therefore seem like it makes no difference using soldiers or zombies, but zombies have several advantages;

-As zombies are effectively 3 soldiers, casting raise dead has the same affect (in terms of atk/def points) as building 2 soldiers, but without costing gold.
-They suffer less casualties than soldiers

- They only cost 2 maintaince.

However

-If your upkeep is equal to, or nearing your revenue before casting "raise dead", the additional upkeep for the converted zombies will put your income in negative values, and you will start losing gold.

My advice would be to use raise dead infrequently. If you use it every time you buy a small number of soldiers (i.e.the soldiers are only a few percent of your army) then most of it will go to waste, but if you wait until about 10% of your army is soldiers, then less of it will go to waste (and you could have cast alchemy the previous times to gain gold).


Another option is to stock up on soldiers, until your upkeep is nearing your revenue (so your income will be low, and convert as many soldiers in to possible in to zombies. You will have more troops than you can financially sustain, but you will be stronger than usual, and are in a better position to conquer enemy lands, and doing so successfully will increase your land, and therefore income, until you reach financial stability again.

(but as I said, I'm not a necro, and zombies may have some other (dis)advantages)

It is also worth noting that a proportion on soldiers that die in combat will be raised again as zombies for free.

Buildings

Mines and barracks

To begin with, you will be spending all (or at least the majority) of you income on troops. If this is the case, then the best configuration for your land is 25% barracks, 75% goldmines. The saving on troops you make form barracks outweighs the gold "lost" from not having 100% mines.
If you are going to be away from your kingdom for a while, (over a day) then it may be worth reading about the cycling method).

Once you army mounts up, and you troop production drops, it may be better to permanently convert your barracks to mines (look in other sections of this wiki for more detail).
The best option may be to not "build" your own troops (so you wont need barracks), but buy them from another player. The problem with this, is that usually they are only for sale in very large sizes, and many smaller kingdoms wont be able to afford (or even upkeep) these vast amounts of troops.

Forts and training grounds

Don't bother. Building them means less land for mines, which lowers you income, meaning you can't afford the upkeep for as many troop. Those troops you can afford will have an increased attack/defence (depending on what you built), but they will pay for it by having a much lower defence/attack (depending on what you built), and the decrease is always bigger than the increase.

The better option is to to supplement your forces with knights or pikemen, depending on whether you want to increase your attack or defence. Any increase in attack or defence will still be met with a reduction in the other, but unlike the buildings, the increase is equal to the decrease.

Magic buildings

Unless you invest a load of your points in mana and/or spell power, and plan to use magic attacks a lot, then don't bother with building magical amplifiers. If you do plan on using magic attacks a lot, then you should have been a magician.

Magic barriers are useless when you're a small kingdom, but when you get to a big size, you will probably get fireballed every so often, so it is a good idea to build 25% of your land as barriers.