You could record with just a USB mic, but an interface will give you a number of advantages, such as recording direct ("dry") signal as well as with a microphone in front of your amp. Each method has it's advantages and disadvantages. The entry level for a two channel interface meets a common price point of around $160. At that price, there are 3 excellent choices.
- Focusrite Scarlett 2i2
- Presonus AudioBox iTwo
- Presonus Studio 24c
All three offer similar features and honestly, any one of them will get the job done. If I were shopping, Since you are planning on using Studio One DAW software, that tips the scales towards the Presonus products, as both interfaces come with a copy of Studio One Artist, which as a stand alone product is $99.
The Focusrite 2i2 comes with ProTools | First, which is a free offering from Avid/ProTools (basically at the same level as Studio One Prime).
The Presonus boxes also are equipped with MIDI In/Out connectors. This isn't as big an advantage as it once was, since most new MIDI equipment uses MIDI via USB. With your background on piano, it's probably inevitable that you'll end up with a keyboard, if you don't already have one. I happen to have a few devices around old enough to have actual MIDI connectors. For me, that would be the final deal maker. If you don't it may not have any importance.
The Sweetwater links for all three products show a comparison grid towards the bottom of the page. Any of the three links will give you a direct comparison against the other two interfaces. You really can't go wrong with any of them. My personal preference is for the Studio 24c.
The comparisons also show the Presonus AudioBox USB96. It's $60 less money, but it doesn't support line inputs (higher level you would get from an amp line out) and only supports mic and instrument levels.
Once you get the interface set up, options for mics are limited only by your budget. A good, all around mic for voice and/or guitar recording is the Shure SM57 or Shure SM58. These make a good starting point and may be all you ever need.
If you have more questions regarding recording and gear, post in either the
Gear Talk or
Recording Tips and Tricks sections and you'll get no shortage of opinions!