The basic skills we use above all others are in the Magic Skill tree. Most of your advancing with Granny'll be Magical skills, and most of the ones you'll use from day to day will be magical as well--unless you're a fighting witch who does little else. To advance in the Witches' Guild--in other words, to grow yourself new warts--you'll need to advance your primaries. Your Guild Level is the average of your level in these skills, which I have highlighted in the list below. You grow a new wart every ten GLs.
A few words about advancing: Granny's knowledge is unquestionably vast, but she's willing to teach a daft wette henne only so much. And she charges--in Lancre money only. Check out the Wette Hennes page for tips on getting some. I've heard if you have gold/silver etc from the Sur areas, she'll take that, too, preemptively even, though I've never caught her at it. Anyways, it takes Experience Points (XP) and Lancre money to advance with Granny, and she'll only teach you to level 10, I think, in non-magical skills, level 50 in non-primary skills, and level 300 in primary skills. Beyond that, you're on your own. You can TM, make a trick, or learn from other players rightly enough--but Granny figures you're smart enough to figure that out once you've reached her limits. When learning from someone else, you'll want to find someone with a higher unstated bonus than you in that skill. Most people advise finding someone with around double your bonus (again unstated, as stat adjustments do not affect the amount of XP required to learn from someone). The number of XPs you will need to learn a skill from someone depends on the difference between your bonuses; a single point difference in bonus can mean at least a hundred XP difference in cost.