Law schools are primarily interested in your overall academic record. There is no pre-requisite coursework, nor do law schools prefer students who follow a specific course of study. A wide range of acceptable majors can be found in the humanities, social sciences, natural sciences, engineering, and business. With that in mind, what is most important is to find an academic area that appeals to you and one in which you will do well.
Admissions Committees will consider the rigor of your academic program and prefer applicants who challenge themselves with a broad-based, liberal arts curriculum that trains them to analyze, read, speak and write well. Upper-level seminars and/or graduate-level courses (including courses open to undergraduates through Penn Law School) can further enhance your transcript and demonstrate motivation and a willingness to take intellectual risks, not to mention that you are capable of demanding work. A Senior Honors Thesis is also a great idea and will very much enhance your academic credentials and complement your application to law school. Admissions officers tend to be aware of transcripts that are "padded" with several introductory-level and/or pass/fail casses and evaluate those applications as lacking in rigor.
If your major is heavily technical or quantitative (for example, students in the School of Engineering and the Wharton School) it is often a good idea to enroll in some writing-intensive and/or humanities classes to demonstrate that you have pursued a well-rounded course of study and can read, write, and conduct in-depth analysis with proficiency.
It is common, and perfectly appropriate, to take a few courses in Legal Studies at the Wharton School to gain exposure to the study of law and to help determine whether you are interested in pursuing a law degree. You should be aware, however, that law schools believe it is their role to teach you law, and pursuing a broad and well-balanced undergraduate education is of the utmost importance. In other words, "pre-law" courses are useful in that they introduce you to broad legal principles and may aid you in deciding whether or not you want to pursue a legal education. However, they are usually not taught with the same depth and rigor as in actual law schools. Take Legal Studies courses because you are interested in them, not as a means of improving your application.
In addition to reviewing your course selection and major/minor programs of study, law schools are also interested in any trends in your grades. Some allowances for an anomalous "off" semester are usually made when substantial improvement in subsequent semesters occurs. In many cases, an upward trend is evaluated more favorably than strong freshman and sophomore years followed by a less than stellar junior and/or senior year. Likewise, if your grades improve after a change of major or leave of absence, this will also have some bearing on the way law schools interpret your transcript.