Note that "tight" doesn't necessarily mean "tense." You need a fairly narrow and smoothly flowing "jet" of air, directed over the top of the hole so that it strikes, and is split by, the far edge of the blowhole.

Many fifes don't have the fully developed hole found on flutes, so it's hard to predict whether yours may require significantly more air than a decent flute. Some do need a bit more, but "blowing really hard" isn't likely to help. It's mainly a matter of an even flow aimed fairly precisely at the right point on the mouthpiece.

I don't have a saved site for referral, but a Google on "flute embouchure" should find pictures that may be helpful.

Once you accomplish "making a tone" it should be pretty much like a pennywhistle.

John