Improve your reading and writing skills. The most important skills for a historian are their reading and writing skills. You'll need excellent reading comprehension and an ability to clearly communicate through writing in a professional tone. You can improve your reading and writing skills by reading challenging texts and writing whenever you get the chance.
Improve your reading skills by looking up words you don't know. Write them down in a journal so that you study them later.
Improve your writing skills by writing letters or emails back and forth with a family member, such as your mother or grandmother.
Learn multiple languages. Historians all need to speak more than one language, as a general rule. This is because the documents being written at the time were often written in different languages because they came from different sources. You'll need to study from these original sources in order to learn more about history. Even as a native speaker studying the history of your own land, often you may need to read the work of diplomats, explorers, and immigrants in order to get a more complete picture. You can learn these languages cheaply from local community colleges, at the university where you get your degree, or even using online study tools (like Rosetta Stone or DuoLingo).
For studying ancient history, you'll often need to know French, German, and multiple dead languages for the area you'll study (Attic Greek, Latin, Middle Egyptian, Sumerian, etc.)
For studying medieval or European history, you'll generally need to know Latin, French and German, as well as some languages like those used by the Celts or Old Norse, depending on your area of study.
For studying North American history, you'll generally need to know Spanish and French.
Note: learning the languages prior to entering graduate study is often required.