The skill training situation with the Spearman is similar to the other class D melee mercenaries. Training the skills which are most useful and easy to come by early in the game is a preferable choice for class D melee mercenaries, due to the fact that these mercs are ultimately outclassed by other mercs which become available as your main increases in level.
It is recommendable to only train Lightning Spear on your Spearmen until you replace them with more suitable mercenaries, unless you have a specific low to mid-level PvP strategy which would require training one of the other skills. Ultimately, you will find yourself wanting to trade out your Spears for tougher mercenaries, and Lightning Spear really is the Spearman's best skill. Additionally, Lightning Spear skill books are easy to come by, making training that skill pretty much the most winning option of the four.
The Spearman is a great unit for certain early game formation builds, specifically those with large numbers of Spears and Gunners, and Lightning Spear is the ideal skill to train for these builds as well.
The more advanced class D melee mercenary skills are useful for PvP at any stage of the game, but the mercenaries themselves are quite weak compared to later class C melee mercenaries. However, if you make your main a melee mercenary, you can bring these useful skills, such as Guard Dispel and Break Down, with you into the late game without having to fill one of your slots with a weak mercenary. Taking a look at which main character classes are at the very highest tiers in PvP, you will see that Axemen and Artillerymen are currently the mains in all of the top builds.
Therefore, if you want to bring Guard Dispel and Breakdown into the late game with you, it is highly recommendable to make your main an Axeman. Freezing Axe is another indispensible PvP skill, giving you another great reason to choose an Axeman as your main. I do not recommend choosing Spear for your main, as the Lightning Spear skill is really nothing compared to Freezing Axe.
For stat increases, the best choices are Strength, Vitality, and Dexterity. You want your Spearmen to deal out the maximum possible damage, and to live as long as possible. They are not the best tanking class, though they can take a few hits and keep on ticking. Spearmen are most useful in those penetrating damage Spear/Gunner builds that we mentioned earlier, and if you are using such a build you'll have your Spearmen up front, tanking. In this case, it is definitely a good idea to increase your Spears' Vitality and Dexterity. But again, I personally do not recommend using Spearmen unless you are using that particular build.