[Lurper]

rb4b, you don't provide enough information to give you a decent diagnosis.
I am assuming by your choice of firearm that you are intersted practical shooting more than bullseys shooting.
That being the assumption, let's start at the beginning. Grip, stance, sight picture, trigger press are the beginning. I could write volumes on each, but will give you enough to get you started in the right direction.
The overall concept of grip and stance is neutrality. No undue tension in any direction.
Grip:
Your grip should be loose, firm and relaxed. You don't grip a gun like you squeeze a lemon. The "gripping" action is caused by your weak hand pinching the grip between it and your strong hand. One of the ways I demonstrate to students is to grip a magazine the same way I grip a pistol and have them pull the magazine out of my hands. They are always suprised at how easy it is to take the magazine. Squeezing the pistol tightly only causes the muzzle to tremble.
Place the pistol in the web of your strong hand as illustrated. Notice the line drawn straight from the pistol through the center of the web and down the arm. Don't let it get off center. Grip the pistol as high as possible, the idea is to get your hand in line w/the axis of the bore as much as possible. Notice how the gun looks like it is pressing into the hand. Keep your strong hand thumb as high as possible - on 1911's keep it on the safety on your XD you'll have to work with what you have (don't let it interfere with the slide). From the strong side it should look as it does in the 2nd photo.

The weak hand should fit perfectly in the area left open by the strong hand. You will need to rotate your weak wrist forward so that the weak hand thumb is as high and a bit in front of the strong hand thumb as shown in the 3rd photo. The reason you rotate your wrist has less to do with grip and more with stance. It keeps your stance (the upper half) neutral by making your weak arm bent the same as your strong arm since the gun will be centered in front of your strong side eye. Where most people run in to problems is by putting pressure on the gun with their thumbs. Your thumbs should just lay there and do nothing (like my ex-wife). Don't push up, down or sideways, just let them rest.

Trigger press:
The most common problem is putting too much finger on the trigger (particularly for those who learned on DA guns). For SA guns, put your finger in your mouth and bite it halfway across your finger nail. The line that is created by doing that is as far as your finger should go. DA guns will be a bit more because of the length of the trigger pull. You will notice that I use "press" to describe the action. That is because it is more of a press than a pull. Right now, we are going to stick with basic concepts, I am not going to teach you to prep the trigger, follow up, etc. For now, you just want to make certain of two things: 1. that you press the trigger smoothly straight back 2. the only thing that moves when you press the trigger is your trigger finger. Some people have a tendency to "milk" the gun when they pull the trigger. That is when their (usually) entire hand moves as they press the trigger.

Sight picture:
Sight picture is easy. With iron sights, focus on the front sight. With dots, focus on the target. Your eyes can only focus on one object, so the target will be blurry. But, it really has more to do with your mind than your eyes.
Learn to track your sight and call your shots. Tracking your sight means that you have it in focus during the recoil cycle (or when transitioning from target to target) it is what allows you to shoot incredibly fast - another advanced technique but one that you can practice now. Calling your shots means knowing where the sights are when the shot breaks. I don't have to look at the target to see where my shot hit, I can tell where I hit by watching the sights. This is an absolute fact: THE BULLET HITS WHERE THE SIGHTS ARE! It can't happen any other way. If you can't say where your bullet hit, then you are not watching the sight.

Stance:
Your stance should be natural and relaxed. Stand in a fashion that would allow you to stand comfortably all day. Face the target, your feet will typically be about shoulder width apart. Also, typically your weak side foot will be slightly forward of your strong side foot. I can't post any more photos on this post, so I will do my best to describe it. You should be relaxed and standing up straight. Don't lean forward or back. There should be as straight a line as possible running down from your shoulderblades to your butt cheeks to your heels. Most people have a tendency to lean one way or the other. Keeping your shoulder down and your head up, raise the pistol to your eye level (as opposed to lowering your eye level to the pistol). Your shoulders should be relaxed. Your wrists should be locked. Your elbows should be relaxed, bent and unlocked (they will work like shock absorbers and convert most of the upward travel to rearward travel, again an advanced technique)- this is what allows you to control recoil and shoot multiple shots quickly. The main thing is to relax. One final tip:
Natural Point of Aim:
To find your natural point of aim, stand in a proper stance, point the pistol at the target, close your eyes, twist left and right at the waist (not your feet), stop when you feel comfortable. Open your eyes and see where the gun is relative to the target (laterally, not vertically). If the gun isn't pointing at the target, adjust your FEET so that it is and repeat. For example: if the gun stops to the left of the target, slide your right foot back or your left foot forward.
That should be enough to get you started.