DC to DC converter: How do I convert 110 Vdc to 5 Vdc with 2.5 Amps current?
For that kind of step-down ratio (110:5) you need a flyback or other coupled inductor based SMPS topology. This is not the simplest of topologies especially as it involves the transformer. This kind of flyback is of course very common so you can find one off the shelf. 

As others have pointed out, 110VDC is no laughing matter. It's considered safe for "instructed people" according to IEC 62368 classification (ES2) but it's not harmless and that kind of voltage can have bad consequences (fire, electrical burns) otherwise as well.

Where are you getting 110VDC from btw? If you have a rectifier for 110VAC it's actually 155VDC, even worse. 

I wouldn't suggest this as the first electronics project unless you've got a supervisor.