The common pitfalls and things to pay attention to here are:
keep your hips even, meaning the hip of the front leg should not stick out to the front; gently pull it back and bring the other hip in one line with it. This adjustment should also help find the proper alignment of the pelvis.
press into the outer edge of the back foot; that foot is not paralleled to either sides of the mat, but is somewhere in between, slightly rotated. Actually, this rotation of the foot allows the propers hip alignment.
Pressing palms together vs keeping arms paralleled in Warrior depends on the style you of yoga practice.
In Vinyasa, we keep arms paralleled and gaze to the front to keep the neck long. In Ashtanga, we are pressing the palms against each other and the gaze is directed to the thumbs. If you're not sure which variation to go for, I'd recommend to start with the Vinyasa one which is more neutral. If the arms go all the way up while the shoulder blades stay lowered, this is a good sign to try the pressed palms version. The Ashtanga version is more intense. But at the same time I feel it prepares you better for backbends, opening the shoulders a bit more and giving this extra spine extension.