If you've got a bike, you need a lock. Whether you are just wanting to deter someone from walking off with your ancient pub bike, or trying to keep all but the most determined thieves from stealing your treasures, it's going to take more than a helmet strap clipped around your wheel.
Sadly, if your thieves come armed with angle grinders or are the sort prepared to cut through shed roofs or garage walls, there's probably not a lot you can do except slow them down. So for those scenarios (and it's not as unusual as you might imagine) it's important to make sure your bikes are insured and that you are storing them in a way which meets the terms of your insurance company. Often you will find that this means locking your bike to a fixed object (not another bike) with a Sold Secure Gold standard lock.
Sold Secure is an independent tester of security products, and most locks will advertise which rating they achieve. As I've said, most locks can be compromised eventually with an angle grinder, though some will take longer than others to get through. But before you say you want me to attack these locks with drills, hammers and grinders, I suspect that the experienced thief is going to know the best way to compromise a lock - me poking at it with a drill or whatever without that knowledge isn't really proving much, though perhaps it might be entertaining. A professional ratings schemes like Sold Secure doing the independent testing is likely much more reliable.