A non-official time will have been recorded by a coach or parent using a stopwatch who may or may not have been trained in timekeeping. An official time will have been taking by at least two trained officials or using an electronic system and will be logged online....
If you weren’t given this when you joined the club, you will find it on the Swim England database https://www.swimmingresults.org/individualbest/ (search by surname). This is also where you will find all your official times from licensed meets.
If a competition will accept non-official times (level 3 and 4 galas and internal club galas) then use times achieved at club gala’s or micro-league competitions and if these aren’t available, ask your coach for a time trial or estimated time. If you don’t have...
If you are too fast you will need to talk to your coaches to identify higher-level competitions to enter.
Short course events are in a 25m pool. Long course, a 50m pool. You will start out at short course events. There are online converters for short to long course times but you’re unlikely to need those for a while.