Building a wardrobe is a long process which can seem daunting at first but over.
In order to get a sense of what to buy and wear you need to consider factors like
- Your personal style and self image you try to present
- Occasion you are dressing for (casual streetwear, semi formal school/work, social outings, etc)
- Location you will be in (outside, bright/dim lighting, potentially getting dirty, etc)
- Comfort (can you stand/walk a lot, do exercise/large movements, cold/hot weather, etc)
- How the clothes fit and sit on your body to achieve balance
- Emphasis created by layering and adjusting clothes
- Colors used to match or create pretty contrast
Remember that you need to feel comfortable in your clothes and presentation to be confident and it's normal for this to take time and money spent on experimentation. This means you need to practice, try on clothes and be able to visualize what would and wouldn't look good as part of an outfit.
This girl has a common body type with narrow hips and wider shoulders and she gives good tips on what to think about when picking out items for an outfit https://www.youtube.com/watch?v=klKVm1FALhs