Since you are still a beginner i suggest that you use a new set of strings and adjust it to the guitar standard tune to let you know how your guitar sound in the standard pitch, and to know how new strings sound it well in terms of tone stability and brightness. This will greatly help you in your quest to learn how to tune the guitar because old strings may not give you the right sounding condition and may not help set your tone hearing capability perfectly.
Guitar Tuners
There are various ways wherein you can tune your guitar. One is through the piano keyboard (see illustration below)
Second, is through the online guitar tuner
There are many available guitar tuners online. You can always check there if your guitar still sounds in the right harmony. It's pretty helpful for us if we from time to time hear those standard string tones on guitar tuners in order to sink it deeper in our memory.