OK - as a rule, you should buy from a reputable site - because, as advised, there are a lot of touts out there who will rip you off.
The chances are the gig will sell out - BG are a huge live attraction - so don't wait too long.
If you think you are going to be a long way back - and if it's the Arena, you can be a LONG way back, then take some binoculars with you - you'll be glad of them because the sound will be fine, and you need to be able to see to increase your enjoyment.
Most major bands have large viewing screens either side of the stage at stradium gigs, so with your biocs, you;ll see fine.
Leave plenty of time to get there - train is best because the station is right by the venue, but if you are driving, remember there will be a huge influx of traffic, and parking is at a premium, so get there as early as you can. Coming off the venue car parks can take up to an hour, so have a flask and some butties waiting to pass the time.
If you are on the train, take a jumper and coat with you - you can take them off inside, but you'll be glad of them when you come out.
Don't take food or drink with you - you will have it confiscated. if you want to buy food or drink in the venue, it is expensive, and there are big queues.
If you need any more advice, re-post - have a great time!