I'm wanting to start growing and I was wondering if any ordinary person can grow? Is online the only place I can get seeds?
If you dedicate yourself to learning, yes. If you think that if an "ordinary" person can grow then it means that you can without putting time, effort and resources into it, then you might grow something but your harvest will suffer in quality and quantity, if you get any at all. Assuming you are in the USA, unless you know someone personally who could get you seeds then online is your only viable option.
Could you give me an estimate on how much it will cost to grow? I'm a little tight on money right now
If you want to do at least an OK job, I'd budget for somewhere between $75 and $150 for your first grow if you are trying to do it on the cheap. It all depends what you may or may not have already and how many corners you feel like cutting. For my first grow it ended ip costing me about $150 after buying lights (CFL's), nutes and soil and making a DIY grow enclosure. I expect you could shave a good $50 off if you decided to buy cheap soil and nutes and maybe another $25 if you already have a little closet or other type of area to grow in.
It's really pretty easy at its most basic. Like Toasty said it just requires effort, like any other worthwhile endeavor. If you're willing to do some research then put the work into it sure, anyone can grow. Just don't expect to toss some seeds in some dirt, water once in awhile, and get a brillant harvest.