Nice to meet you! I love to go on hikes. Hiking is what we call going for a long walk on trails in the country. At least that's what we call it in the US and in Canada.
I've always thought about hiking but am unsure where one would go to hike.
You can go hiking on any trail, or even off a trail through the countryside. 
Would walking up mountains or on flat lands constitute a hike. And how long would you have to walk for?
As long as you want. However a "day hike" is a hike that you do in one single day. Some hikes are even longer!
Whats the best hike you've ever taken? And where was it?
I personally love to hike at The Ovens national park in Nova Scotia. You can see beautiful scenery when you hike, and it's known that all forms of walking have health benefits. No wonder it's a popular activity worldwide.