I haven't heard or read anything bad about booking.com, but I usually try to book directly with hotels so they don't have to share my payment with booking.com.
Like the first poster, I start with flights, defining the start and end of the overall trip. Early flight booking can mean serious savings and a better selection of flights and seats. We usually get the cheapest non-refundable tickets we can, taking schedules and connections into account (fewer connections = better, avoid northern US cities in winter). Then we spring for "economy-plus" or whatever the airline calls it for the transatlantic segment. If we have to change plans, as we did this year, we pay change fees. The prepaid fare is still usable on different dates.
Hotels come later, after more research. Usually they let us cancel up till shortly before our stay. We might take a saving on a non-cancellable reservation if the place is in high demand or we're staying so long the saving is significant. But there are many hotel choices and we don't want to get locked into one unless there's a good reason.
As for trip insurance, we usually forego it except when booking a whole tour plus airfare. That kind of trip means big outlays well before traveling, and we see insurance as useful protection. We've had to use it once or twice. This kind of insurance covers your losses if you have to cancel a trip for specified reasons, notably health. It may also cover medical costs overseas if you don't have other coverage. Generally you have to buy the insurance within a short time after first booking the trip. This website has good information and comparisons to help you shop if you decide to go this route. This link takes you to Rick Steves' advice, which is good.