How a new partition Function apply for Current Data

I have a heavy database , More than 100 GB only for six month .every Query on it takes me along time and I dont have enough space to add more indexes.by a wayI decided to do partitioning. I create a partition function , on date filed and all Data records per month was appointed to a separate file.I'ts done and ready. But who to apply this function on current data? And is partitioning only for Future data entry?thank you