What type of diving do I want to do?
Think about your current scuba diving and where you would like your scuba diving progress to, it doesn't have to be a plan, just something where you would like to be - especially in the next couple of years - Air diving, EANx (Nitrox), gas-switching, decompression, tri-mix, semi-closed, fully-closed rebreathers, wrist/watch/console style?
Do I want to log my scuba dives and post analyse the scuba dives?
Then you would require a scuba dive computer that has a download / PC interface.
Do I want to measure current gas consumption on my scuba dive and have information recalculate my remaining scuba dive time?
Then your scuba dive compute will need to be a fully integrated dive computer console, or have the ability to receive data from a transmitter.
Do I want to use my scuba dive computer as a watch outside of diving?
Then it will need to be a wrist-watch type scuba dive computer.
How much money do I want to spend on my scuba dive computer?
Set yourself a budget and see what is the best dive computer you can buy. If you can afford, after you made the selection, add another £50 / €50 and see what you can buy, what additional dive features do you get - it often adds more flexibility as your scuba diving progresses. The last thing you want, is having to replace your scuba dive computer when you do the next course. Your dive computer should give you years of scuba diving support.