LoveMyGirls I am a professional artist and have always dabbled in painting/being crafty generally but have been painting and selling seriously for the past four years. The easy part is the painting, the hard part is to sell, especially at the moment!
The wooden bits that come with the canvas are canvas wedges/keys to tighten up the canvas if needed...you push (or lightly tap with a small hammer) them in the slots in the corners of the wooden frame (try and youtube/google if you are not sure how to do as is difficult to explain exactly how they go in!) :-)
£25 for a 24in square canvas is WAY too cheap. I am a real believer in keeping original art affordable but you have to strike a balance, no one will take your art seriously if you charge that...they will wonder why it is so cheap (or what is wrong with it) and it will put them off. If you can see yourself taking painting at all seriously in the future you do not want to have paintings you charged £25 for lurking around.
Finding places to sell: for any online galleries (or the likes of Folksy) I would say you need a good body of work so potential sellers can have faith that you are professional and also see you have an individual style. Etsy: OK but you can easily get lost in the thousands of listings. Dollars can put people off. Folksy: I think is improving, not great at marketing so think a fairly high percentage of viewers/buyers are other craftspeople. One way these can work for you are if you market your shop yourself ie hand out business cards etc.
Galleries are very hard to get in to, they will want a good selection of work and sometimes a proven sales record. Some of the higher ones can charge up to and over 50% commission. There are some smaller galleries around that are easier to work with but I do not know where you are located to advise on that.
Anyway, to prevent me rambling further, here are my best tips for you:
-Develop a distinct style/theme. Don't just paint lots of random paintings to sell (or do but just for practice!). You want to be easily recognised through your work.
-Keep a record of sales/expenses now. It may really take off and they will be needed.
-Don't approach galleries until you are really confident and ready. If you do and they turn you down you will have wasted that opportunity.
-When you are ready start a Facebook page and use it like a mini art blog. It will encourage interest especially if you link form an online gallery so potential customers can follow you daily.
-Paint a wide range of canvas sizes to suit different budgets. I sell small 5" X 5" for £35, the big ones are £100's and not everyone can afford or has the space for them.
materials: Loxley Gold are widely used (and I used to use them) however they are not the greatest quality. Look at the corners of the canvas...they are cut off rather than neatly folded under. The wood used is also quite light/flimsy and some of my larger paintings using these have warped (30inch sq and larger...so annoying!!!). I now use Art Discounts own brand chunky canvases. Slightly cheaper and sturdier in my opinion.
Use the best quality paint you can afford. get some decent brushes...they will make a real difference.
Phew...sorry to ramble! My dh just asked if I am typing an essay 
Last tip...enjoy painting! The better you do (selling wise) the more stressful it becomes. That, however, is a whole other post 