Well, you're miles ahead of most on the net, since you can at least spell "amateur".
It is entirely normal for 100% so-called "crops" to be indistinct or blurry in some way. The key is to compare one to another over time and get a feel for what is "right". That is, if you even need to bother viewing images that large. Remember that this is a totally artificial way of looking at pictures. While it might be useful for some tasks, it is not the be-all and end-all.
The single best way to know how large you can print is to print large. Take a shot you really like and try it at several sizes. See what works.