Need to work on notes?

Well, Im 13 years old, and a guy. I love music and singing. But only REAL music(60's,love Beatles). So for most of my singing, I just sing along to Beatles tunes and it seems to be working out great, every week my voice sounds better. But Im having problems higher up in my range(I usually sing Pauls parts/songs, which are generally the highest). In songs like All My Loving where he sings near the end "All my loving, oo ooo" I sorta have problems hitting the "oo" part. Im pretty sure its in my range, as I can sing it without little problem, but it sounds absolutely terrible. What used to be high for me started out like that when I tried to sing things that used to be what I considered really high, but now I can get them easily and it sounds good. Also, I have problems hitting the "woos" in songs like She Loves You, but I am sure that its within my range. After I try hitting high notes like those, the high notes below them that I can usually hit quite easily start to sound really bad. Do I just need to practice those notes until I can sing them strongly? Please help.