If you wear jeans casually then a shirt with darts and not too long in the body looks smarter than an oversized t-shirt, emphasises your face and neck and hides your stomach.
Peplum tops can be good for hiding a tummy but when you try it on hold your arms up to check that it's long enough to hide your stomach.
Jeans and trousers should come to your natural waistline, if they cut you off any lower then you'll have a 'muffin top' and feel even worse about your stomach.
Not Your Daughters Jeans are a good fit again, apologies if you know this already they are pricey but worth the money NYDJ
Gap do a range of styles and sizes, go on a quiet weekday if you can and try on everything you like.
What are your good points? If you've got good legs or great cheek bones highlight those.
If you don't like dresses on you have you tried different styles? Skater dresses that drop from the bust, rather than the waist might look good, or drop waist dresses.
If you're anywhere near a John Lewis their personal shopping service is good, friendly and not intimidating.
As a change from trainers shoe boots or ankle boots are comfy and smarter than trainers and they go well with jeans or dresses.