Using HD monitor with 2011 Apple Macbook Pro
Your machine will support up to 2560x1600 on an external monitor.
However, using a converter thunderbolt to HDMI will output max of 1920x1080. If the monitor you are planning to connect to your laptop is HD(720p) or Full HD (1080p) then you are good to go and can use a converter thunderbolt to HDMI. If the monitor's resolution is higher than 1080p then you will need a minidisplay port to display port adapter(mini display port is the same thing as thunderbolt, but it is cheaper to find it looking for a mini display port). Make sure that the monitor has a display port, otherwise you will need to use dual DVI cable with an apple adapter, which is well overpriced and may be considered as an overkill. Hope this helps.