Learn Russian: Раньше я интересовался русской историей. - Earlier I was interested in Russian history.

No, it is not a tool. However, the verb интересоваться (to be interested in) always requires the thing you are interested in to be in the Instrumental case. You can think of it as keeping yourself engaged by means of Russian history.

Both words are feminine Instrumental, so why does русской end in -ой but историей ends in -ей?

They follow different spelling patterns for the feminine Instrumental case. The adjective русская takes the standard hard ending -ой to become русской. The noun история ends in a soft -я, which changes to the soft ending -ей in this case.

How do we know the speaker is male from the word интересовался?

In the past tense, Russian verbs change based on gender. The ending -л tells us it is masculine. Because it is a reflexive verb, it also has the particle -ся attached. If a woman were speaking, the verb would be интересовалась (adding an -а for feminine, which changes the reflexive particle to -сь).

Could I use до instead of раньше to mean "before"?