You don't have to decide your area of law in your first year. You don't declare a major, and your law school does not care if you ever choose a specific practice area. The only reason you have to choose one eventually is because your EMPLOYER wants to know which practice group you want to work in.
Most people decide after the summer of their second year (because if you work for a law firm in the second summer, you will usually get a permanent job offer from them for after graduation, and they will ask you which department you want your offer to be from).
However, some students don't decide until the third and final year of law school (either they work for a different firm/organization than they did in their second summer, or they ask their summer firm to let them change their department).
Also, some people change their practice area after a few years of work. You don't have to have studied that specific area in law school; you actually learn a lot about the law "on the job."
There are many types of law. The two main areas are litigation (lawsuits) and corporate (mergers/contracts/negotiations).
However, within litigation there are many kids:
intellectual property litigation, products liability, malpractice, commercial litigation, personal injury, etc.
And depending on how big your law firm (or government/nonprofit office) is, each of these kinds of litigation might be its own department, or it might be just one "litigation" department that does all these things (or does a few of these things--not every law firm practices every area of law).
Similarly, within corporate, there are many areas, which could each be its own department or could all be in the "corporate department":
tax, mergers & acquisitions, intellectual property (patent filings), trusts and estates, etc.
Or some law firms take some of these areas out (e.g. "tax" as its own department) while combining everything else into either "litigation department" or "corporate department"
To get an idea of how many different practice areas there are, you can take a look at the list of practice areas that the law firm Skadden, Arps has. I say Skadden because it's the biggest law firm in America, so they have more practice areas than almost anyone else. But do keep in mind that there might still be some areas that Skadden doesn't do, which other firms do:
In terms of lucrativeness, in general you are paid the same salary by your law firm regardless of which department you are in--your salary as an "associate" is only based on seniority and experience (plus a bonus if you do well) for the first 8-10 years, and then you can try to make "partner," which is a group of senior lawyers who manage the firm.
So lucrativeness for you as an individual doesn't matter that much in practice area. But trusts and estates is one of the most lucrative areas for the law firm, because rich clients pay big $$ to get their wills done. Similarly, intellectual property work can bring big $ to the firm as well (patents on new drugs, etc.). So, if you have a specialty in either of those areas (or something similar), it might be easier to get a job sometimes (because the firm will profit more), but you won't necessarily be paid more.
At the top 200 firms, starting salaries are all $150K+ per year for new associates, with raises each year, and partners can make from $400K to $2 million or more.
At smaller firms, salaries can range from $40K to $150K starting, with raises each year. And partners' income depends how profitable the firm is--among smaller firms, there is more variation in profitability.
If you work for a nonprofit organization, or for the government (Attorney general's office, DA's office, SEC, FTC, etc.), salary generally ranges from $40K-$70K at starting, and maxes out around $100K-$180K after a bunch of years. So you make less money. But, you work less hours than law firms do, because government offices are open for shorter hours (just like the post office closes way before the FedEx store closes--it's the same with government lawyers vs. private law firms).