Advance voting has started for the 2017 election. Yes, that's right - you can vote from today! Personally, I prefer the party atmosphere of election day, but if you want to do it early, you can. And no, you don't need to give a reason.
Emma Hart has a guide to the process on Public Address here. And if you're not sure where to go, just pick your electorate here and the Electoral Commission will tell you.