Connecting Dell U2715H to 13" MacBook Pro (mid 2012)
Try using an HDMI to HDMI cable (or other) to check that the combination cable you are using isn't broken in some way.

I have a new (mid-2017) MacBook Pro 13" with touchbar and found that certain combinations just don't work when I connect to the same version Dell monitor. For example I bought a USB C to HDMI cable link here, and it will not work. I'm not technical either but believe it or not, there are different USB C settings! The website now says their new cable doesn't work with MacBook 2017 models, after I informed them. They said that they will now produce one that will.

Therefore, if changing the cable  (borrow one from a friend for an hour) doesn't cure your problem then you may be experiencing similar issues to mine.

Additionally, you now have information to avoid my mistake if you buy a new MBP and use your Dell monitor :-) 

Hope this helps.