How can I connect an external monitor to my MacBookpro

You need an adapter---which one depends on which MBP you have. You should have received it when you bought your computer but if you don't have it you will need to give us the last three digits of your serial number so we may identify which model you have. Basically I believe there are two models of adapters--late models use a Thunderbolt port and early models use a DVI port.

If you look at your computer and see a DVI port that's the type you need--if a Thunderbolt port a Thunderbolt adapter.

A thing to watch for is there are full size and mini DVI ports so the serial numbers would be a big help.