It's better for a Necromancer not to focus on attack, but rather on defense and health, so that his troops take less damage. It's worth taking Education right away; the first combat spell should be Ice Bolt, as it not only deals damage but also freezes. Meditation is needed, in my opinion, if you're playing on a huge map against a human opponent, closer to the end of the game, so to speak, for the final decisive battle. Again, it's worth remembering that meditation consumes 3 mana points. Subjugation makes an enemy unit (except those immune to mind control or magic) yours for one turn, meaning you can't attack it. It doesn't lose initiative, as it did in Heroes 5. Any damage dealt to the subjugated unit will break the subjugation. You can use the unit to attack neighboring units or to draw an enemy unit away from your units. The subjugated unit blocks attacks from neighboring enemy ranged units; they usually wait, then someone from the non-ranged units attacks the subjugated unit, thereby breaking the spell, although they will receive a counterattack. The effectiveness of Waves of Renewal depends on the number of Priestesses; they have two charges. Waves of Renewal removes all negative enemy spells from the unit it's cast on for three turns. Moreover, Renewal from one Priestess can remove the most powerful spells, and also restores its hit points up to resurrection; the more Priestesses, the more health is added. It's very effective to cast Waves on a creature that is linked to Stream Spirits; then they will also start to "renew." In practice, Dance of Thorns only slows down opponents that can be slowed down; for example, it doesn't work on Dryads or Angels.

How much experience does a Hero get for the next level? Like 5000 or 10000, or how much?

It increases gradually. Go to the hero's profile; there's a scale in the upper right corner that shows how many experience points are needed to reach the next level.

Added 5 minutes later

I've been thinking about it a bit.
Education should really be taken on the first turn, as it gives +20% experience.

Education gives +25%