You can use a primer, touch up with powder or try a moisturizer for dehydrated skin or one that's oil-free - or if you don't feel you need a moisturizer at all in the heat, don't use it at all or use only on the dry areas of your face.
For an 18 year old girl, I'd go for something like a MAC 217 brush (£18) or the new Mineralize Skinfinishes in the Tropical Taboo collection. The colour called Soft and Gentle is supposed to be very nice - it's £22.
You could also go for an eye shadow palette from Sleek (sold in Superdrug and other places). Some of them have been used by Pixiwoo, and are £7.99 for a palette with 12 colours.
An Alexander Wang ruched skirt would be cool with converse shoes. There are loads of cheaper alternatives around.