Section 1 - Recommendations
Windows: Zynewave Podium Free
This DAW is quite basic, but it definitely gets the job done. Simple and straightforward. It comes with almost nothing, but following the link on the website, you can find many free VSTs for your instrument and effect needs. There's also the list here!
You can easily use this for mixing covers, making midis for USTs, and writing original songs. The free version also comes with permission to sell any of the songs you write with it.
This comes preinstalled on all Macs, so if you're a Mac user, you're already familiar with this music making app! It's possible to install your own AU plugins to use within Garageband, so you're not limited to what's made available to you.
I've never used Linux myself, but LMMS is quite a lovely DAW on other platforms. It comes with many instruments of its own and is very similar to FL studio, as well as being used by many people in the UTAU community already.
Section 2 - All resources
GarageBand (Comes with OSX)
Studio One Prime
Now we're getting into commercial DAWs. Read about editions and features carefully before making a purchase.
Less than $100
You might as well be a professional