If you have gotten accepted in colleges, BUT your first semester grades aren’t great, BUT not bad… will anything bad happen???
Depends on the overall dip, if you go from an A to C then that may pose an issue, but if its A to A-, or even B+, I think you’ll be fine. Try to maintain your grades as best as you can 
I wouldn’t do anything until I was asked about the drop in grades. Colleges will get your grades for the last term and may ask you what happened. If your grades have been consistent or improving, the school may ask what happened or make the expected decision based on how closely you match admissions requirements.