You should start with the basics;
- Processor Information\% Processor Time
- Memory: Available Bytes
- Logical Disk*\Average Disk Queue Length
- SQL Server:Buffer Manager\Buffer cache hit ratio
If you went with the default install of non-express SQL then it will take all the memory it can and starve IIS/Windows. On the other hand, if SQL doesn't have enough memory it will have a low Buffer cache hit ratio and you will probably see disk queueing. Most other counters will be a side-effect of a constraint on disk/memory/processor for a single server installation. Once you get to the point where you need to determine if it is: IIS/SQL, php vs asp.net, identify intensive WPs, etc. then the monitoring becomes more complex and you will need to do a lot of research or hire a contractor.
User performance monitor or advisor tool linked at the bottom to setup a data collection schedule to log the key performance counters. Generally a 5-15 minute sample is sufficient for overall system monitoring/analysis. If you are trying to diagnose a specific issue you may go down to 15s or less for short periods of time.
Pages/Sec: Average of extended periods > 150 can indicate a problem, but it also depends on disk. On a cloud server you might want to avg < 150 as the disk is likely shared or remote. Also look at Available Bytes, if pages/sec is high and Available Bytes is high, then the counter can be misleading as there are other causes than paging that are beyond the scope of this question
% Processor Time: If it averages or you see extended periods > 75-80% you are CPU bound.
Buffer cache hit ratio: Generally should be 96-99%, lower indicates that SQL is not effectively caching and needs more memory
Average Disk Queue Length: Log for each individual disk, > 2 indicate that the disk is saturated. You can also log PhysicalDisk\% Idle Time and if it falls below 20% it indicates disk saturation.
You can also use Microsoft's performance advisor to jumpstart your monitoring and have more thorough monitoring based on Microsoft recommendations;
Microsoft Performance Advisor