Don't feel like you have to suddenly make massive changes all at once. You don't have to suddenly start wearing the most fashionable trendy clothes you see, small changes can be a good place to start.
In an ideal world, if you knew nobody was going to judge you or think anything negative at all, what kind of things do you like or would you want to wear? What style jeans do you wear now, for instance, and is there a medium in between that and wearing skinny jeans? Or with t-shirts, could you maybe try a polo shirt one week, and then build up to trying a shirt and jeans? Or what about 'accessorising' your jeans/t-shirts a bit - belts, a scarf as it begins to get colder, some kind of wristbands, a new watch, or some different shoes?
How is your relationship with your parents? If your mum basically chooses your clothes, could you say to her that you've been thinking about what style suits you, and see what she thinks would suit you? She's probably buying clothes that she thinks you're happy with, so would probably be very happy to choose some different things with you.