There are really no rules. As far as financing it: In theory it would be nice for your parents to pay for your wedding like they used to do, but some families can't afford to. Basically if you (or your fiance) don't have parents that are willing and able to pay for it, it is going to fall on the two of you. If neither one of you can afford it, you will have to decide if a "wedding" is really important to you. If it is then you will have to save up and have a wedding when you can afford it. If it is not, that is how lots of people get married at a justice of the peace. Ultimately, the responsibility is on you.
As far as planning goes: You can ask anyone you think would be a good source of help. Your mom, best friend, whoever you think would be able to put in some time and wants to help. You should defintely give your fiance some say in things, but usually the guys don't care nearly as much as us girlls do about the details. Moms are usually the ones wanting to be the most involved. I started planning mine with a few bridal magazines- they have timelines you should follow and checklists, they are lots of help.