Buy stuff you know you are going to use and that is for your skin. Don't buy things just because they're cheap or trendy, think about what you're buying. You can search on TikTok, Pinterest and more for recommendations for makeup products for your skin and what's good to use.
When trying out eyeshadow put it on your knuckles as it is kind of similar, then you will see if it's good. Use your neck to see if the concealer, foundation and more is your shade as it will then look more blended in with the rest of your body.
Also I would look at makeup tiktokers that have kind of similar makeup look as you want or the same skin type as you, so it is easier to get ideas on what you want to do or get recommendations on products.
My fav tiktokers are:
Swancullens(Because of their colourful looks)
Trevorbarrett(Has really good tips and easy to understand)
Linsmakeuplooks(Has really fun and good tips)
Hope this helps!!!