There are many possibilities for performance problems. The main issues are CPU Load or disc load. Where to look first would depend on what the system is used for. if a server is used as a file server for a company network i would check Disk I/O first and monitor CPU load and network I/O, too.
Database server have usually high CPU loads.
If your server lacks memory and has to page a lot you can see that the memory usage is beyond the physical memory, in this case best is to upgrade memory.
For a very quick peek you can just open the task manager and watch the CPU load for a while on the server. If the system idle process gets below 50 % too often you have high CPU usage. In task manager you can add additional colums, you should add the disc I/O (read write) columns too and watch which processes access drives a lot. The networking tab gives you a good idea of how much your NICs are used.
if you want more you can use the microsoft performance monitor, so you can log performance over a longer time period. The performance monitor wizard helps a lot, you should download it if you plan using the performance monitor and dont have much experience with it.
http://www.microsoft.com/downloads/details.aspx?familyid=31fccd98-c3a1-4644-9622-faa046d69214&displaylang=en