What position?
Figuring out what is overactive (short) and what is weak, and correcting the imbalances will make the biggest improvement on flexibility and mobility.
As for hips, most athletes should focus on their posterior kinetic chain - glutes and hamstrings. What is your resistance training history like? Basic things like double and single leg supine bridges, swiss ball leg curls, and glute ham raises are a good start if you don't have much training experience. Moving forward, rumanian dead lifts (single and double leg), good mornings, barbell hip thrusts, hip pull throughs, kettle bell swings, and trap bar dead lifts are all good exercises. Once you get a handle on these you can move into clean pulls and hang cleans.
Br