Firstly I have to admit that the last trainers I bought were about 15 years ago and were more 'fashion' than 'fitness' trainers (Adidas shell-toe ones). 
I've decided to join a gym and try and tone up a bit but need some trainers and, TBH, there are so many around I don't know where to start.
I'm likely to do weights machine type exercise, maybe some exercise bike/rowing to warm up rather than running machine (knackered knees).
I can get to a Sports Direct and JJB to try on and buy.
I'm in my mid-40s so please don't recommend anything too youthful that will make people laugh at me 
Can someone point me in the right direction please.