• Good first spells : Curse and Luck.

Jotun militia is probably the best in the game. For 15 GP, you get a Jotun Herse and 5 Jotun militia, and for each level of defence you get an additional 30 HP Jotun militia. After 20, however, you only get wolf riders, which are not worth it. 19 militia and a Herse is a fairly effective fighting force (for 190 GP), and only large armies will be able to defeat them.

Units:

In general, Jotuns are fast and *really* hard to kill. Even if they face a horde of smallish units and break, most of them will still survive, while the smaller units will take serious casualties even if they win.

If you have the full version, you might want to try adding some vaettir or wolf riders to your order of battle, to help keep your Jotuns from being surrounded. Local troops could work for this too. I think it may just be that Jotuns are in a big rock-paper-scissors: their high HP and protection and good AP allows them to easily crush archers, but hordes of light infantry can surround and overwhelm Jotuns (surrounded troops have a defence penalty). To the other hand, the giants are so big they can't themselves overwhelm much of anything to get the defence reduction.

Jotun Spearmen are probably the best flat-out infantry in the game. Their armor is quite high (18), and their HP are triple the average troops. Also, while their attack and defence are only average, their strength is so high that they usually kill even well armored troops with one blow. Their weapon length is 4 (behind only Pikeneers), so they are quite good at repelling anyone. Jotun Axemen do slightly more damage, but the negative to attack is painful, the negative to defence is bad too, and their reach is only 2.

Jotun Hurlers are very potent second line fighters. With their strength of 20, the boulders do a total of 40 points of damage, which effectively kills any normal troop even with high protection. The short range is actually a bonus: this allows them to hit much more frequently. Some screens of Jotun spearmen, with a group of Hurlers directly behind, are excellent at taking out almost any force. This is even effective versus swarms of Knights as the Hurlers ignore their high defence values (range attacks ignore defence), and usually kill them in one shot. Remember that close combat giants can’t benefit the surround bonus and so tend to have problems hitting high defence units.

Woodsmen have a missile attack too. They even get one more missle attack versus the hurlers, plus they are much better hand to hand fighters (but less protected than axemen or spearmen). Hurlers are stronger versus super-high armor units. Woodsmen are better against medium or low armor, and will fare better in long fights where they are forced into hand to hand.
However Woodsmen are one of the worst units in the game for a cost ratio - if their armor was better they might be worth it but as it is they die far too easily compared to Spearmen, and should be considered as special use troops.

Your home province has a special site that will give you one free Longdead Giant if you have a death mage visit it. Get a Siethkona, select her on the main map, and then click on the province special. Longdead Giants are not overwhelmingly powerful, but is a decent bonus early on.
When Jotunheim mages cast the spell reanimation about half the undead are giants too. So this is a much better spell for them than for other nations. When they cast raise skeletons/dead in combat about 10% of the summons will be giants. It's not amazing... but it's something extra.

Seithkona and Norna are relatively cheap but very low HP mages. They are good for researching, and after evocation has been researched a bit, are incredibly effective support mages due to the spell Nether Darts. For only 90 GP, a Seithkona with a small infantry/undead screen can usually disperse dozens of troops with Nether Darts, and also is frustrating to enemies as they will likely be left feebleminded by this spell (even with the cheaper version, Nether Bolt). This is a great way to booby trap an arena deathmatch. Unfortunately, their low hit points make them vulnerable (they are not protected from cold BTW) so don't get too attached to them. If you have a very powerful Norna (good Hall of Fame bonus, lots of experience, etc), you can cast Transformation on her and probably will get a mage who will survive much longer. Note that any equipped items when you transform will disappear.
Furthermore they can all help with communions, which can lead to some very impressive spell casting.

Skrattis are expensive, but their skill in water magic is useful. Furthermore, blood magic can be very profitable: put a Skratti on find blood slaves, as soon as you get 5 slaves make a dousing rod, and then keep him finding slaves. With a decent growth rate, your population will increase anyway, and a single Skratti with a dousing rod in a populated area will generate 5-10 slaves per turn. Blood has some very effective summoning spells, particularly the summons that create monsters that can create monsters for free from then on: Bind Heliophagus, Bind ArchDevil, Bind Pazuzu, Curse of Blood. Many of the monsters these summons create are flying, which is of great use to Jotun.

With Skrattis, Jotun can become a fairly large underwater threat. Create hordes of undead, particularly through Carrion Reanimation or the special Longdead Giants. In battle, having the Skratti cast Quickness, then repeatedly summon Lesser Water Elementals is a good general aquatic battle plan. Make sure he has bodyguards.

Jotun Herses are the only sacred leaders who haven't priestly powers and so don't require a temple to be recruited. They can be blessed and their upkeep costs is less than the troops they're based on. Of course you can't replace all your Jotuns with Herses but you can often buy 1 Herse / 3 Spearmen in castles where you'd otherwise recruit 4 Spearmen and can't or don't recruit a magical leader. They are also expendable - a perfect choice for a early death arena : the almost guaranteed Hall of Fame bonus can be huge.

Magic:

Jotunheim's magic is quite unflexible, since Nornas have only random sorcery picks. So no Wards and no big air/earth/fire spells even with a Communion (the latter lets you cast Poison Ward and higher sorcery battelfield spells however, like Relief, Charm, Polymorph, Mass Regeneration, Battle Fortune, Doom, etc -- but you probably prefer to see your mages cast Nether Darts instead).
However, Jotuns definitely have a big advantage when forging magic items to enhance commanders, since giants are much tougher combatants than most other leaders. Nornas can forge relatively cheap equipment that can turn Jotun Herses into fearsome warmachines: Wraith Swords, Lycanthropos Amulets or even Barkskin Amulets are very effective to keep them alive and healthy. And if you have access to earth mages, Boots of the Behemoth on a few size 4 leaders can wreak havoc in the enemy ranks.