im newish to the fute andi find it very hard to get anynote lower than f

what am i doing wrong, plz help

If you are brand new to the flute, that could be normal. Or your flute could have a problem.
Do you have a teacher or the possibility of consulting someone more experienced to have the instrument tested?

you may have a leak in one of your keys, or it could b just because your quite new to the flute
my little sister can't get notes lower then f, as she has only been playing a couple of weeks

I had the same problem when I started playing the flute 15 months ago. Now I can get all the way down to low B. I use a wide embouchure to spread the air and I blow rather soft from the diaphram. It takes a strong embouchure to get the low and high notes so it will take time.

In addition the causes already mentioned it may also depend on the flute headjoint and how it is cut. I have Pearl Elegante and it has a wonderful headjoint for the upper register with relative ease up to C4 but low C and B took some practice to get. Seems like headjoints cut so that low notes are easy are more difficult to play in the upper register and "versi vici". But as skill increases this becomes less of a problem- I still have work to do. Control of air stream force (less force) and directing the stream a bit (wee bit) more down into the embouchure hole helps for me. Good luck.