Hopefully I'm going to run the first session of my new campaign tonight but I still havent decided what level to start the characters at.

The main villan will probably be a green hag or bheur hag (depending on what level I start it at) with a few minions/pets of a similar CR (such as a Leucrotta or Barghest) and she will have several mundane and weird magic items. What would be an appropriate level for an encounter like this?

Thanks in advance!

Depends on how long you want your campaign to last. If the party aren't planning to fight the main villain until a few sessions in, you can have them start at level 1 or level 3. Otherwise if this a one shot or they're fighting the villains straight away, you might want to have them start at level 5.

How experienced are the players. More new players = lower starting level. Newer folks need some time to learn their powers and spells, so it’s tough to dump a lot of options on them at once. 

A group of level 1 players could take out a Green Hag if that's the only thing they're doing that day... if it's at the end of a short dungeon that involves a lot of fights, maybe they'll be too spent by the time they get to her... but they also might just level up half-way through. I'd say if you've got level 1 characters, have a plan in place if the party all falls. A green Hag is intelligent enough to have use for a group of living adventurers... she might capture them and take their stuff away, so the players aren't completely FUBAR thanks to a few bad dice rolls and you don't have to come up with a completely new adventure for next week.

To me there are only a handful of levels worth starting at

1 - long term campaign where the characters journey matters and you want to give them a chance to naturally develop quirks and motives

3 - long term campaign you want to jump start into more action quicker

5- one shot or short campaign where you want to have access to more interesting enemies

7+ one shot

17+ power fantasy

for this, 4 or 5 people should be lvl 2, 6+ lvl 1, 2 or 3 should be lvl 3 and 1 person should be lvl 5

Quote from sardonicmonkey >>

To me there are only a handful of levels worth starting at

1 - long term campaign where the characters journey matters and you want to give them a chance to naturally develop quirks and motives

3 - long term campaign you want to jump start into more action quicker

5- one shot or short campaign where you want to have access to more interesting enemies

7+ one shot

17+ power fantasy

 

 I like these levels and basic summaries as a solid rule-of-thumb.