For everyday people shooting.. EYE's! Everything else can be slightly blurry, but if you get their eyes you might still have a good shot.
Get a 50mm prime lens pronto. 1.8 or below. You can pick them up dirt cheap and they take great people shots (especially faces).
For the type of shots you are posting, it's more about lighting than anything (which can get expensive), composing your model and having a good background. You have a lot of harsh lighting going on in the shots and they are not very sharp. Also, the backgrounds are pretty bad. Place your model further away from the backdrops, step back some, and zoom in on your subject. This will likely blur the background some.
Ladies tend to like long lenses as it compresses the image down and makes them appear slimmer.. that's why the 70-200mm f2 lenses are so popular.
Shoot heavier subjects from the top down to slim them down, and place heavier people in the center of group shots if you using even a slightly wide angled lens.
If you are on a budget, check out collapsible reflectors (
https://www.google.com/search?q=came...w=1680&bih=886) Even if you only have one camera flash/strobe/light source, you can use them as fill light. Use these during day light shots as well, to fill in the harsh light.
Built in flash is always horrible.
In low light shots and you need to use a flash , get a speedlight with a swivel head, rotate it backwards so it points behind you (or points to a wall or something that will reflect light).. bump your ISO up to 800, F4-5.6, and it will dramatically improve indoor low light shots.
You can also buy (or make your own) beauty dishes as well.
If your camera has the ability, you can remotely fire off multiple speedlights to create some cool effects or properly light a subject. Get some cheap umbrellas and light stands.
Once you get the hang of lighting a scene/subject, look into getting some Einsteins or other high power strobes with some pocket wizards to go pro.
http://www.paulcbuff.com/e640.php
Finally, visit sites like
http://digital-photography-school.com/. They have a ton of tutorials, photo challenges, tips and tricks in their newsletters, etc.