OP, I've done a turnaround you are planning to do over the past 1,5 years. Nowadays, I get complimented daily for my looks.
I'm size 10-12, 5ft8.
Where I started out:
Ill-fitting jeans worn with boring black tees.
Ugly practical shoes.
First step: I found a pair of jeans that really fits. i went through my old jeans, found a pair that I really liked, bought a new and updated version of it.
2nd step: I bought some bright tops to get rid of the black. Red looked best.
3rd step: bought some dresses and opaque tights
4th step: I thought skinnies don't suit me but finally found a pair that did (I have to wear either ultra tight superskinnies or cigarrette pants. The inbetween style does not look good on me.)
5th step: I figured out that the placement of waistline makes a real difference for my looks. Too low, muffin top and jeans fall down. Too high, proportions don't look right. Midrise was a good compromise but because I have very long legs I look best in dropped-waist outfits. Bought a few, they made a massive difference.
6th step:got hair sorted out. A sharp haircut and highlights made a huge difference.
7th step: had my colors done. A crucial step, gave me the confidence to try bold bright colors. Love red, coral, emerald green. Denim blues and blue gray and navy are my best neutrals.
8th step: spent a lot of time on Makeupalley and gradually revised my makeup - brighter colors, better products. A good lipstick is the cornerstone of my style.
9th step: lots of shopping, trying on, returns, trying on. Sorted out shoes (wedges and ballerinas suit my everyday lifestyle), found my most flattering shapes and styles.
So as you see it has been a long and gradual process. If you can arrange some child-free time to go browsing, that might help - I found shopping with DC impossible. Maybe you have a friend or relative whose style you admire? Take her along? Don't forget charity shops and jumble sales. Swap clothes with friends. I found a very stylish jean jacket last night for 3 euros. If you look at style blogs, you can also see a lot of good looks built with Primark pieces. If you have a birthday coming up, you could ask for a color or style analysis or personal shopping appointment as a present (it will be an investment that will yield long-term results).
Finally, watch the video recommended on the "frumpy" thread. I'm sure you'll get lots of ideas from it!