So that's your shoes sorted, but when it comes to what clothing to wear hiking, you can look at it like you would when going into the mountains in winter. Think layers. Three-layers of tech if possible. Layered clothing enables you to add or shed layers according to the conditions like changes in temperature, precipitation, wind and your activity level.
First and base layer:
The most important of all your layers because it's in direct contact with your skin. Avoid cotton as it absorbs moisture and holds onto it keeping it next to your skin, so go for a technical fabric that's breathable and 'wicks' away moisture. The fit is usually quite snug to ensure the right amount of heat is retained. Most are made of synthetic materials or Merino wool, which is a great fabric for a base layer because it will keep you cool when it's hot, and warm when it's cold. It has unbeatable wicking properties, and being naturally antimicrobial it doesn't get smelly like man-made fabrics. On days when temperatures are moderate to cool, we'd recommend Merino base layers, but for short hikes in hot temperatures you might be better off with a lightweight polyester base layer. And on hot summer days a base layer on its own is likely all you'll need to wick away any moisture keeping you cool.
Brands to look for would be SmartWool, Patagonia, Helly Hansen, Black Diamond, Rab.
Second layer
The importance of a mid-layer cannot be overstated and throughout summer a small fleece or light down jacket will likely be all you need, depending on where you go. Look for a good level of breathability, something quick drying and not too bulky. The material should have a good level of stretch and in some conditions a gilet is a better option than a long-sleeve. Like with base layers the materials can be both synthetic and/or natural.
Brands to look for would be Arc'teryx, Peak Performance, Patagonia, Black Diamond, Salewa, Rab.
Third layer
This is your shell and shields you from wind and rain, and in many ways is your most functional (and likely expensive) piece because it needs to be waterproof and breathable, which often means paying for tech like Gore-Tex. They are important when the weather suddenly changes because if wind and water penetrate to the inner layers you will get seriously cold. Look for taped seams and weatherproof zips to ensure the rain stays out. Depending on the weather, the outer layer doesn't necessarily need to be waterproof, and some soft shell jackets provide good wind resistance and can be more breathable than waterproof hard shells.
Brands to look for would be the same as those mentioned above.
Trousers or shorts
If you're unsure how the day's weather will pan out, think light hiking trousers (some come with zip-off bottoms), and pack some lightweight shorts in case of warm weather. Like the layers on your top half, hiking trousers should fit comfortably and feature quick-drying tech.
Socks
A pair of quality Merino wool socks is your first line of defence against blisters and tired, sweaty feet. Read any hiking socks reviews and Merino wool is the standard-bearer, and whilst they can be pricey they are tough enough for any trails and are built to last. They are extremely comfortable, wick moisture well, breathe, regulate temperature and importantly take a while to get stinky, meaning they're great for an extra day of wear.
Something worth noting is that even Merino wool hiking socks aren't ever 100% Merino - you'll generally find them around 30-80%. Density varies, which is important for shape and durability. Generally sold as lightweight, midweight and heavyweight, for summer you'll be fine with lightweight, the only downside being they are thinner and offer significantly less padding and blister protection. In terms of sock height, crew height is the most popular which extends about 6-8 inches above your ankle. Quarter height is over the heel and ankle but not up the leg, which is good if you're wearing hiking shoes or trail running shoes. Many companies sell both men's and women's specific socks, but in most cases there are no major performance differences between the two. To keep your socks looking fresh year after year, wash in cold water and line dry.
Depending on how long you're hiking for you may wish to pack a spare pair of socks and a spare base layer for when you want to freshen up.