You don't specify, so I'm assuming United States. You also don't specify a time period or focus. You'll need that.

The first step is applying to a program. You want to go to the best program you can manage, because we live in a world where going to a prestigious school still carries some weight when it is time to get a job.

To apply to a good program, you need a strong application. That means good grades in undergrad, good GRE scores, a good writing sample in your target field, good letters of rec from relevant undergrad professors, and a good range of relevant coursework. If you are going for ancient history, for example, you will want to have substantial Latin and/or Greek as a baseline, as well as some demonstrable experience in a modern language of scholarship like French or German or Italian, the more the merrier. This is field-dependent. Having an idea (or sources?!) for a dissertation is not required to apply to a phd history program in the US. That comes later. You will first need to complete coursework and a gauntlet of exams. In my program, I had quals exams for Latin, Greek, French, Italian, German (shudder), three written field/topic/domain exams, and then Orals. That's a good 4-5 years, if you work your ass off. Then dissertation prospectus. That being said, it never hurts to have an idea of what you will work on, and with whom you would like to work. You should apply to a program, though, not a professor, except in unusual circumstances. In Europe, things work a bit differently, and I can't speak to that.

Expenses: when you apply to grad school in the USA for liberal arts, you are also applying for funding. If you are admitted without funding, you should not go (under any circumstances). You will dig yourself into a hole of debt, and at the end of it, if you are lucky, you'll have a phd in history. Then you'll go to coding boot camp and wind up working as a code monkey for Groupon so you can pay back your massive debt. No funding, no attending. Funding typically can take the form of fellowship (= free money that you get and may or may not pay tax on, disbursed either in one giant lump sum or else per semester/year) or else teaching (you teach classes, they pay you a pittance); or a mix of the two. If you don't come into grad school rich, you will suffer. You will get an insanely small amount of money to live on, and life will be very hard if you live in a place like Boston, Philadelphia, Manhattan (lol), Palo Alto (bigger lol), or Berkeley (biggest lol). I think conditions might be much better in Europe as far as funding.

The workload will depend on the program and the field. In general: a lot of work. You will have a series of exams to pass no matter where you go, some less, some more, and you will possibly have Orals or a Defense in some form or another, which varies by program. You will read pretty much every book and article that has ever been produced on or related to your topic.

The best way to find out more about this is to go and talk to a professor in your field. Ask them to give it to you straight.