Powering DC 5v 120w from AC 110v
There are a number of ways of doing this, with tradeoffs for efficiency, ease of design, cost, space etc...  But if you're new to electronics, then consider buying something like this:

LS150-5 - AC/DC Enclosed Power Supply (PSU), Compact, 1 Outputs, 130 W, 5 V, 26&nbsp;A - from Farnell UK

This has the advantage that all the (dangerous) mains electronics is done for you and bundled up in a box, so there's less chance for mistakes. This particular one is from the UK, but you should be able to find one wherever you live.  The trick is not to look at ebay or amazon, but at an actual electronics supplier. In the UK, that would be Farnell, Rapid, Mouser or RS. In the US, you might look at Digikey.