The most important thing when choosing a flute is to get one that is suitable for your hand size. A flute in the key of A or G is a good size for most people.
The hand sizes for flutes overlaps a little.
Our general recommendation is:
From the smallest flute up to F# is suitable for small hands.
From A up to Low C is suitable for medium hands.
From F# up to the largest flutes is suitable for large hands.
The larger the flute the deeper the tones. The smaller the flute and the tone will be higher, as they are in a higher octave.
If you're playing on your own you can play any key, it only matters if you are playing with other musicians as you would usually want to play in the same key.
F# is the most traditional sounding. In general for a beginner a flute in the key of A will be a good size and medium sound.