I am new to this myself. I have a Brewer's Best American Cream Ale kit in the carboy fermenting as we speak. I have two issues that came from this first batch that I am looking for help with. Maybe these are good topics for the OP as well so you don't repeat my mistakes

First, I think that I filled the carboy up too high after I added my wort, because foam is bubbling through the airlock. Is that normal and will the beer be okay if i just replace it with a new, filled and sanitized airlock?
Second, this sounds really stupid, but the one thing I forgot to purchase was a hydrometer. I feel like the beer should still be close to what Brewer's Best estimates the ABV to be at, but is there a way to figure out the ABV without the OG? I may not worry about it on this one since these kits seem to have a high success rate. Thoughts?