To do Warrior I, step one foot forward with the toe pointing straight ahead as you step the other leg back with the toe point out. You want to set up in a nice wide stance so that you can sink down into a lunge with your front knee over your ankle. Make sure the back toe is turned out so that your feet are about perpendicular with your front foot lined up with about the instep of your back foot when it is turned out.
Then bend your front knee and sink down into a lunge. Keep your back leg straight and the front heel firmly planted. Make sure your front knee is open and not caving in. Rotate your hips to face forward as you reach your hands up overhead. Squeeze the glute of the back leg to make sure that you are extending through your hip while keeping that back leg straight.
Also, do not rock to the inside of your back foot. Keep that foot fully planted. As you hold the lunge, try to get your front knee bent as close to 90 degrees as possible and your quad parallel to the ground, but do not let that front knee cave in. Squeeze that front glute to keep your front knee open.
Beginners may not be able to sink as deep in the lunge. You may find you need to step your front foot forward a bit more to sit comfortably in the lunge and keep the front heel down or you may even need to step that front foot back if your knee is behind your ankle while in the lunge.
The lower you sink in the lunge, the harder the move will be. Breathe and relax into the lunge. Make sure you keep both feet flat on the ground and your back knee straight. As you reach up and back, do not hyperextend your low back. Make sure to squeeze your glutes and stretch through your hips.