Often, new guitarists have a very hard time tuning their guitar. Learning to listen to pitches very closely, then fine-tune them, is a skill that takes practice. In teaching situations, I've found some students can't easily listen to two notes, and identify which is higher, or which is lower - they only know they don't sound the same. If you're having a similar problem, try this:
Listen to, and play the first note. While the note is still ringing, try humming that note. Continue to play the note, until you've managed to match the pitch with your voice. Next, play the second note, and again, hum that note. Repeat this - playing and humming the first note, then follow that by playing and humming the second note. Now, try humming the first note, and without stopping, moving to the second note. Did your voice go down, or up? If it went down, then the second note is lower. If it went up, the second note is higher. Now, make the adjustment to the second note, until they both sound the same.
This may seem like a silly exercise, but it does often help. Soon, you'll be able to recognize the difference in pitches without humming them.
I hope this has helped. As previously mentioned, it's extremely important to tune your guitar every time you pick it up to play it. Not only will it make your playing sound a whole lot better, but the repetition will allow you to conquer tuning your guitar quickly. Good luck!