I wouldn't consider what you were wearing before to be "immature".
It's simply a matter of taste. If you want to try a new look to reflect and express who you have become now, then that will be a really positive change and a lot of fun too
Do you want to have a complete image overhaul or would you like to take the style you talked about, and update it a bit to become congruent with where you feel you are in your life now?
There is nothing wrong with T-shirts in general. I love T-shirts. If you are comfortable in jeans and T-shirts you can still wear them but dress it up a little.
If this is what you want to do, I'd suggest investing in some high quality jeans. If you want to look trendy, you can get a fashionable brand for cheaper on eBay (beware of fakes if you are buying something expensive, and ALWAYS get measurements! I am a big fan of jeans, especially sass & bide, and also tsubi, but these are both Australian brands.) For basics, I'd recommend a dark, plain pair with a straight or skinny leg that you could dress up or down, and a more casual medium wash in either straight or bootcut. Definitely get them hemmed if they are too long, as scuffed, frayed, dirty hems can ruin an otherwise perfect outfit. When you have the money, I'd also recommend you buy a pair with detailing that you love, so they can be your "special" jeans that will make you feel special each time you wear them. They could be distressed, or have unusual detailing on the pockets, or a different wash- whatever you like.
Embellished tees are a more chic alternative to the faithful old T-shirt. sass & bide make some gorgeous ones with a bit of quirk.
Here are some example of the kind of thing what I mean:
(It doesn't have to be sequinned. Choose ANYTHING that appeals to you!)
For the warmer weather, you could try a cute bolero over your tee, either plain or something pretty and detailed.
A fitted blazer is a more sophisticated and grown-up alternative to a hoodie or zip-up fleece jacket (don't know if you wear these though).
Imagine an embellished tee with a fitted blazer, well-fitting, high quality jeans, and cute heels, flats or boots- still a tee and jeans but a lot more dressed up. You could even wear plain tees with these items for a more casual look. For something different, try different styles of tops- buy inexpensive ones when you are discovering your style so that if you find that they aren't "you", you won't have wasted a lot of money.
For those more formal occasions, find a well-cut pair of plain trousers that fit you perfectly- probably either black or charcoal (or even dark brown), depending on what suits you and your taste. Team these with your more dressy tops.
If you're new to wearing skirts and dresses but are willing to try, start with a classic little black dress (or another colour you like if black isn't your thing) and then branch out to different styles when you develop more confidence and a greater sense of what you like and feel good in.
When following trends, I'd suggest buying cheaper accessories and tops to save money that you could spend on more classic pieces. Eg. you could wear a plain tee with a trendy statement-making necklace or scarf and you will look trendy even though you are wearing a plain top. Definitely steer clear of trends that don't suit you or that you don't really like though! Style is much more important than trendiness!
Don't forget to dress for your body shape! There's plenty of info about this on the net or in books like Trinny and Susannah's "Body Shape Bible".
To help you choose colours, I highly recommend Leatrice Eiseman's book "More Alive With Color".
Please let me know if this helps at all, as I am studying to become an Image Consultant
