throw the old one away, don't worry about how long it is.
get your bike upside down, position wheel forward in the drop out, as far forward as your tensoiner will allow.
get new chain around your rear sprocket then round the chain ring and see where it comes, break it there with chain tool, these are only £3 in tesco, and join it with it's quick link, tension if needed, then enjoy.
PS: i got chains 1/2" x 3/8" 116 links, more than enough, complete with quick link for joining in Red, Yellow or plain silver
£7.49 posted to your door royal mail first class.