Well, you can delete files you absolutely know you will not need anymore.
However, the better option is to go here: https://skydrive.live.com/options/ManageStorage and buy additional storage so that you can keep all your files plus more at a low cost per year.
I think the best option for you is the $10/yr plan. However, when you do ever need to create space, only delete files you know that aren't needed by you in the future.
Let me know if this helps or if you have any questions. Thanks.