This section lists the best guild choices for new players. This does not mean that you should not consider any other guilds beyond these - but you may find the game much harder if your guild is not newbie-friendly.
If you can't decide, don't worry too much! BatMUD has a reincarnation system that allows you to redo your character, at the cost of a (usually) small amount of experience. Almost all players reincarnate at least occasionally.
Have you already joined a guild, only to find out that it does not work very well for new players? No problem - if you are under level 40 and have less than 4 day playtime, you can recreate to rebuild your character, while keeping all your items and experience. If you do not meet these requirements, you can instead reincarnate - see help reincarnate in game.
When planning a reincarnation, a really useful tool is the Reincsim, available at http://batshoppe.dy.fi/reincsim2/statsim.php - here you can easily check whether you have enough experience for a reinc of your choice.
One thing to note is that BatMUD generally follows the "tank / DPS / healer" trinity, and most guilds specialise in one of these. You -can- combine multiple roles, but this option is generally out of reach of new players.
The main "rule of thumb" to remember is that caster guilds, with one exception, don't really work well for soloing - so if you pick one of these, you will be rather reliant on your ability to find a party (or have friends) to play with.
Most guilds offer one or more systems of advancements that are commonly refered to as "reputation" (or "rep" for short) systems - these allow to you gradually increase the power of your skills by actively playing the guild. The reputation systems, and the power gain that they provide, are one of the main attractions of the various guilds.
For information on how to actually join the guild, see help (guildname).
Section 3 of this document includes more information about each guild, including links to existing guides where available.
Rangers are an offtank guild with a variety of wilderness skills. They fight with axes or long blades (short blades work too, but are weaker), and while they get rather boring after a while, they work great for a new player thanks to their solid combat and health/endurance regeneration options.
Bladed fury is an excellent attack skill, and camping + fire building together with the corresponding masteries allow you to regenerate between battle fairly fast.
Note that rangers here do not use bows. Rangers do not have any reputation system outside of the ranger quests.
Any race with a big size should work well. Focus on strength, constitution, and dexterity.
Tzarakk is a mounted offtank guild that fights with polearms. The mount is rather weak initially, but grows in power swiftly and becomes a great asset - it can take hits for you and significantly improves your combat skills. It can also carry loot for you.
To join Tzarakk, you first need to complete the Disciples of Chaos guild (10 levels). This guild lets you to transform to a spawn of chaos that increases your stats and gives you extra attacks, at the cost of reduced experience gain. It also gives you clawed strike - a good attack skill. The spawn is not very strong at first, but becomes stronger as you play with it.
Camping combined with the cure light wounds spell allows you to regenerate fairly quickly.
Again, any race with a big size should work well. The mount helps you wield bigger weapons, so somewhat smaller races should work fine, too. Focus on strength, constitution, and dexterity.
If you are looking for a mage-themed guild, riftwalkers are a solid choice. The guild lets you summon an entity that will tank hits for you, letting you shoot spells at mobs from behind. The guild also offers healing and entity regen spells, protection spells, as well as several nice utility spells such as invisibility and floating disc to carry loot in.
Starting as a riftwalker can be somewhat tricky, as the guildhall is in the middle of a lake and you need to capture the entity yourself, but both are doable as a new player. Don't forget the floating option of your newbie crystal!
Focus mainly on intelligence, then wisdom. Some strength (for carrying equipment) and constitution (for maxhp) is nice as well, though not as important.
