If you are going clipless for the first time (a very good move by the way) you are guaranteed to topple over once. It's a right of passage. If you choose a system with adjustable tension you will be likely to dial them down too low due to fear of toppling over but you will find yourself pulling out of the pedals. That is unnerving. You will then crank it up and as I stated you will topple over once at some point. Again, this is a universal right of passage. You will not escape. Anyone who tells you they never toppled over in clipless is lying. The only thing you can do is try not to use your best bike at first. If you only have one then try riding in the park on grass or something until fate chimes in. After that, you'll be fine. But, you must fall once.