There are a couple of milestones to aim for, and you'll actually see there's a progress bar on your dashboard counting up to them.
First milestone: 100 subscribers: This unlocks ad revenue. When you get views, it will turn into a trickle of revenue. It won't be a huge amount (my comic with over 1000 subs makes less than 20 cents a month from ads as a guideline) and you won't be able to withdraw it yet anyway.
Second milestone: 250 subscribers: This used to automatically unlock the ability to get ink donations and withdraw your earnings to paypal, but I've heard you now need to ask the staff to unlock it for you when you hit this number. It's worth knowing that you can only withdraw earnings when you have $25 or more that was received at least 30 days ago.
In terms of amount, on a comic of around 250 subs, ad revenue is typically a few cents a month, so not enough to withdraw, but comics that participate in the community usually get enough donations at the November "Inksgiving" event to at least be able to withdraw it and get themselves a little Christmas treat!
There are other contracts that may be offered to creators with a higher sub count, but the best way to get them is just to concentrate on steadily building your work's following and being an active and constructive part of the community 
