What kind of an OS are you running and what will be the 2nd OS ?
If they are all windows OS you can install both of them on the same partition as long as you have free space :-) It will modify your boot ini and give you options to login to OS 1 =Xp OS 2 Vista etc.
If you wish to use a partition manager i think you should read the Help file that comes with the partition manager ...Open the program and press F1 on your keyboard and you will have the knowledge base you want.
I prefer partition magic over partition doctor. However if you can get HIREN Full Boot CD it comes with more tools you can never imagine to help you :-)
If you are not good at computers please may you get someone to do this for you . Do what you know and know what you do first before you get into a big MESS