Usually I lurk around and not involve myself too much, but your story resonated with me a lot. My situation isn't precisely the same as yours, but my family is in a huge financial pinch, and I've been trying to push myself to find a job for a long time, even if my mom doesn't want me to.
As much as I'd like to comment on the conduct of your step-mother, I can't, because as long as you live under her and your dad's influence, there's not much that you can do until you can support yourself. One of those horrible "You live under my roof so you have to listen to me" sort of things.
For the job part though, I'd suggest going to your friends or relatives for help first, see if they have any connections or resources you can use. If you've already tried that route, there are other options.
Submitting a resume online is rather ineffective (unless you specifically see the store put up the website)--if you're interested in retail, it's better to just walk into a store and ask them straight up if they need help. Even if they don't, you can hand in your resume anyway just so they have it. If they ever put up a "help wanted" sign, give it to them again.
Also, now that summer is arriving fast, lots of summer programs will be seeking counselors, teacher assistants, etc. If you can find something like that to buy some time, then go for it.
Try going to odd places, like nursing homes, libraries, even restaurants, to see if they have openings. If you're really in a pinch, you can't be picky. More recently, I've heard that blogging is an OK way to make money, but I personally have no experience with it. I've poked around though, and Hubpages and AssociatedContent seem to be great places to start. Don't expect any quick cash though.
I don't think it's intrinsically your fault that no one is hiring you. However, keep in mind that your goal is to stand out from other applicants. Everyone and their dog has "good organizational skills," is a "quick learner," and "a good team player." Putting something like that down in a resume is just asking for a rejection. Employers want to look at a resume and see facts, followed by a unique human being who fits job qualifications.
Change your resume to fit each job that you want to take.
Giving your "sales associate" resume to someone who wants a "tutor" is not the way to go.
Finding a job is hard, especially if you've never held a job before. It helps to read up on tips online (sometimes I skim articles that come up on my homepage). Most importantly, be confident in your skills and know what your skills can do for you. Don't let statistics discourage you. If you're fortunate enough to have a college education, you're already more qualified than 70% of people out there.
... I hope that helped. That ended up being really long. Q_Q
Edit: I wrote this when I was half asleep, so there are some other things I thought about (your situation really bothers me somehow u_u). I'd really like to think that your dad really won't kick you out, since you are his child after all, though I don't know the true depth of your relationship with him. If you can talk about it with him about it, would it help? And I know you and your step-mother aren't on the best terms, but it doesn't do anyone any good if you openly reveal that you dislike her (I don't know this either though). While you're still dependent, I really think there are laws preventing your parents from doing that, unless they want to force you on other relatives or something. It just seems wrong!!
And the other thing is about your grades. I don't think you should treat that as a negative experience. In fact, you improved the second semester because you learned from your mistake and truly wanted to do better. You can use the anecdote as a positive sign of your diligence, especially if it comes up in a job interview, or on your cover letter (I've actually never used one, but a lot of people want them when you apply for jobs..)