Hi there!
As you are already aware there are a lot of sites which do pre-sales or some sort of fresher "deals". Personally, I would suggest waiting until freshers week to get tickets to any events you are interested in. Most societies will only start posting their socials and events a week before or during freshers week so there really isn't any need to worry now. As for the club tickets, the most reliable site is FIXR. They usually release tickets 7-10 days in advance.
When I was a fresher I also thought I had to buy everything in advance and ended up not going to any of the events I pre-paid for because I met other people and saw the events that were advertised during freshers week!
I hope this helps.
Laura
Exeter University Student Ambassador