There are little tricks you can use to find intervals. For example, a 6th interval seems complicated but it's the same interval as the in the first two notes of "Here Comes the Bride" which everyone can do because it's so familiar.

I also cannot overemphazise constant practice as was noted earlier in this thread. Tape record yourself singing and you'll surely hear where you are off pitch. If possible, practice intervals with a piano. If a piano isn't available to you just practice singing along with a recording but be careful not to choose something that's out of your singing range.

Richard