It depends on how similar the lyrics are...if their are specific lines copied then you might want to change them because that's over the line a bit. Also, if it has the same meaning/"story line" (just with different words) then you probally need to change a little bit there. It's okay for lyrics to be influenced by other lyrics, but if someone hears your lyrics and automatically bring up another song, that's when there needs to be some changes.

When it comes to the chord progression, just make sure it doesn't sound a bit like the other song. Even if the lyrical meaning is similar, if the chord progression is different, then it's an original from you.

When I first started writing I ran into this probally also. It didn't take that long for me to grow out of it though (that might be different for other people...I don't know). Once you start writing more, you esablish your own originality. Just keep writing and remember it's okay for your song to be influenced by something else, just don't let it be the same (like the advice I said above..). Good luck!