do i need any sort of lighting equipment? and what settings should i use while taking pictures? ISO? Aperture? does anyone have experience with beach portraits?? i appreciate any feedback. thanks!
Shoot very early in the day if it's an eastern beach, late in the day if it's western to get the sun low over the water, and use it as a backlight, with flash as the main for the "water in the background" shots, then shoot at the opposite end of the day for the "beach in the background" shots with your flash as normal fill, but the sun at enough of an angle that your subjects don't squint. As a general rule, use fill flash to light people outdoors almost every time you can, preferable with an off-camera flash on a bracket so the flash is off-axis from the lens. Personally, I'd shoot on manual with the flash power dialed down from the default matrix balanced fill- but how much depends on the angle and power of the ambient light. Use the histogram or meter spot or center-weighted to get the exposure right, as the sand will bias matrix a bit too much.
Seriously, "I'm shooting some friends at the beach" has significantly different connotations than "I've booked three shoots." Coupled with a question on ISO, settings and *if* you need to bring your own light, it definitely sounds like a train wreck waiting to happen- and the connotation for "I've booked" *IS* that the shots be magazine-worthy.
As far as your samples go:
The first image could have used fill flash to avoid the dark bags under the child's eyes, and avoiding the harsh mid-day light which resulted in the blown highlights and distracting white reflection in the background would improve things immensely. Pay attention to what's in the background, as well as it's tone- the eye is drawn to bright spots, and you really want the eye drawn to your subject. Don't shoot at mid-day unless you can't possibly avoid it. Wait for your subject to look up, and make sure they're not blinking- people can only "connect" well with portraits if they can see the eyes, not having the subject's eyes open works for "moody" artistic shots trying to portray an emotion or action, but not so well for family-style portraiture. A little extra light on the hair is ok, the completely blown out forearm isn't so good.
The second image needs to have some "space" for the girl to look into, and again avoiding the harsh mid-day sun and dynamic range that's nowhere near the camera's capability would have helped. The brighter background elements on the left draw the attention away from the child's eyes a bit too, positioning to the right a bit would have helped with that, but you'd be fighting with the increased color in the background- so overall moving the child and chair over to her left, then shooting with her on the right side of the frame would have been a much better adjustment to make, keeping the muted colors in the background to make her the center of attention. Again, the harsh sunlight hurts, but bringing enough flash power to bring the general level of light up a couple of stops would have helped that from being a blown-out patch. The negative space on the right draws the eye away from the girl's face and eyes, framing with space for her to look into would have negated that and made it a much, much better image.
The last image isn't bad, but the stop or so of difference in the shirt isn't matched by the lighting on the face, which is a tad distracting- but you can fix that by damping down the bright side of the shirt in post-processing. More problematic is the pose- the ears aren't symmetrical enough for the straight-on look to work as well as a slightly off to the side view. I also find that de-yellowing the teeth in post for portraits helps immensely.
As far as ISO, always shoot at base ISO if you can, as that gives you the most dynamic range possible from the sensor, the only time you should ever go above base ISO is if you need the additional speed.
As far as aperture, you should shoot for the depth of field you require, using a depth of field calculator is good, especially if you can work out the distance and field of view necessary for group and individual shots.
Afterthecalm: The only reason for a blown out picture is not exposing correctly- it doesn't matter if you're shooting at 1/125th or 1/4000th, you have to check your exposure, and dial in bias if your metering is presented with a challenge. Beaches with lighter colored sand are like shooting in the snow, and if you don't bias your exposure similarly then you'll get poor results. The histogram on a digital camera is one of the best ways to judge exposure, as well as chimping some test shots.