There are a large number of thing that can cause you machine to boot up slow, here are some I would start with.
Check you startup items and disable the ones you don't need. On a pc: in run or search type in msconfig and click enter, go to the startup tab (don't uncheck processes and apps you don't know without researching what they do, as it make cause additional problems on the machine). On a MAC go to System Preferences > Users > logon Items.
Make sure you have available Hard drive space on your system partition (such as C drive)
Move large files from desktop if there are any, you can create a shortcut to them once they are moved.
If you have a large amount of files on your desktop try either moving them or at least consolidating them into folders.
Run pending updates. Also run a malware scan to make sure there's nothing hidden and malicious causing this.