There's lots of love for Transcribe! and that's what I use. However, some TBers prefer other "loop and slow down" software. Various threads here on TB discuss Transcribe!, Amazing Slow Downer, AnyTune, Audacity, and others. I think all of them require an MP3. As has been mentioned, the Transcribe! website tells you how to create an MP3 if you don't want to buy one (but that's other issue). I buy all my music that I want to listen to. However, some of the iTunes purchases have not been compatible with these programs, so I switched to Amazon for individual MP3 downloads. If I'm just learning it to play it, I use Audacity to record the audio off a youtube video.
Then you still have to use your ears. If you're just beginning, you might have a tough time picking out the bass line. If just takes practice, but try raising the playback by an octave and listen for the lowest notes.
In terms of pitch, identify the key the recording is in by listening and just hunting and pecking on your bass. Then do a google search for the chords, but don't be surprised if you find out that they're wrong (at least to some extent). Having some knowledge of music theory and chord tones helps.
Don't be surprised if the toughest part is rhythm, depending on what genre you're into. I recommend learning to read standard notation to develop an understanding of rhythm. Start with finding the 1 (the first beat of every measure), and then figuring out the time signature. Most tunes are 4/4, buts lots aren't. One of the advantages of Transcribe! (if you end up choosing that one) is the automatic subdivision of each measure into beats. When you fine tune the measure markers, the beat markers automatically adjust.
With enough practice, you'll learn to recognize chord changes, melodic lines, and rhythms just by listening to them. If you spend the time, you want have to rely on just tab and watching youtube videos. Nothing wrong with them, but they're not always available, and when they are, they're not always right. Yours won't be either, but they'll improve over time.