How to monitor load on database server
There are a couple of things here.  First performance monitoring is a subject that is very broad.  When I started down the rabbit hole of performance monitoring I found Brent Ozar and Paul Randal's blogs very helpful.  

Specifically Brent's post on perfmon counters.  Its a little old but still a very good resource.  To go along with that is Paul's post on I/O latencies and Brent's How to Prove Your SQL Server Needs More Memory

You will notice that most of the information I have posted here is from only two sources but they are the first place I tend to look.  The second place I look to is Google.  On top of those there are third party monitoring solutions such as Red Gate SQL Monitor, Confio Ignite (now Solar Winds) and many others.

The way I learned how to do performance monitoring was by being in a situation where I thought something was going on and so I started to try and prove it.  Google was my best friend to find blogs and other information as well as reading this site and others as people asked questions.  Once I got enough information from perfmon and other sources to point out that there was something wrong but we needed better visibility into the server I was able to get a third party monitoring tool which really helped.  Turns out we did not really need more of something just tune several queries.