You can get good yellows. Mostly rings, amulets, circlets, boots, and gloves. There are some decent cheap Runewords to work with. BiS gear is usually a mix of expensive runewords, hard to find uniques, crafted stuff (which is RNG and requires high levels), and some rare items. What's best for the Necro, depends on what your building. Making a Summoner? Then Arm of King Leoric and Homunculus are great. Not too difficult to find, but not easy either. At the start, you can buy decent Wands from Akara that have +2 or 3 Raise Skeleton and Skeleton Mastery. These cost a lot of gold your not going to have at lvl 1, so your just gonna have to save up and sell everything, then come back later. You can use Spirit sword in-between, and some rare Head (their shields). Later you can use a Spirit Shield as well (but these require high Str, 156, which can take awhile to get when you also want decent Life) For BiS weapon, they'd probably throw on a Beast Runeword to give their skeletons Fanaticism. But that is expensive. Heart of the Oak is also good, slightly less expensive. BiS armor for most classes is Enigma, which let's everyone teleport around like a Sorcerer. It's one of the most sought after items in the game, and makes a huge difference for mobility. Arachnid Mesh belt is the best for all casters. A +2 Necromancer amulet with faster cast rate (crafted ones are better, but you need to be lvl 93+ to start crafting good ones). Rare 10% Faster Cast rate rings. Rare tri-resistant boots. Or War Travelers, Marrowalks, Sandstorm Treks, etc. Best helm would be a rare Circlet type with 2 Necro Skills, 20% Faster Cast Rate, and maybe some other stuff like resistances. Otherwise, a Shako. If your making a Poison Necro, you'd want a Deaths Web unique, which is extremely rare. For a Bone Necro, a White Wand Runeword that has +3 Bone spear and stuff on it (hard to find) or just a Heart of the Oak Runeword. Besides the Spirit runes which are pretty easy to get, finding all this stuff is just luck.
Runeword = an item (weapon, armor, helmet) that can be made with a socketed item + the required runes, which carries special effects and buffs that are generally better than other items you find in the game White items = plain items that can be socketed and turned into a Runeword Blue items = magic items that can come socketed but can never be used as a base for a runeword like a white item Yellow items = rare items that can come socketed but can never be used as a base for a rune word, usually of better quality than magic or white items Green items = set items. These items are collected into a full set to give extra bonuses than what they already do Brownish/gold item = unique item. Much like runewords but you don't make them, they are generally much better than magic or white items Orange items = crafted items that are made with a blue/magic item + 1 perfect gem + a rune + a jewel + horadric cube. Usually made by players who are endgame level 88+ (usually 92-93+) Some good starter items to look out for are white wands for your necro that give plus skills to the tree you are using most. Lots of people like raise skeleton, skeleton mastery, or poison and bone skills. Low level items that will help get you to 30+ would be Stealth Armor runeword, White wand runeword, Lore helm runeword. Look for rares that give you resists. Look for unique wands and armors that you can use until you get endgame things. Try to collect the "Infernal" set at a low level. Trang-Oul's set is the best set you can collect for a necro. Use an Act 2 Merc. Some of the most desirable items for a necro are White runeword, Spirit runeword, Heart of the Oak runeword, Call to Arms runeword, Enigma runeword, Magefist unique gloves, arachnid mesh unique belt, Aldur's advance unique boots, War Traveler unique boots, Bul-Kathos wedding band ring, Mara's Kaleidoscope unique amulet. These are just some things you can aim to achieve for your late/endgame character. That being said, I have no idea if you're playing online/offline, ladder/non-ladder, sc/hc so take my information as you will. Some runewords are not available to certain playing types like ladder/non-ladder. I wish you the best with your character! Hope this helps :)