Face facts: It’s time to start over.

Your music collection is scattered all over the place. You have too many CDs that you don’t ever listen to. Some of your best stuff (and your parents’ best stuff) is only on cassette, vinyl or – yikes – eight-track tape.

But you received an iPod for Christmas, so it’s time to start downloading songs.

So many to choose from. Where to begin?

How about perusing this list that offers some key building blocks for anyone’s popular music collection?

Don’t expect to see classical sonatas or obscure jazz choices on the roster. These are major releases from the previous 40 years that are available at any mom-and-pop music store.

Even though I could comfortably expand the list to 500 songs, I’m limiting my choices to 20.

But if you want to add something by Queen, The Police, Johnny Cash, Stevie Wonder or Nirvana, I’m scarcely going to protest.

A Day in the Life

With so many Beatles masterpieces to choose from, this one may be the most all-encompassing choice. It perfectly showcases the yin and yang of Lennon and McCartney’s voices, and it celebrates the harmonious and discordant with equal measure.

Baba O’Riley

Still the best rock song of all time, The Who’s 1971 effort also ushered in a new era of technology by being the first radio track to explore the rhythmic possibilities of synthesizers. Looping and sampling owe a debt to this powerhouse cut.

Whiskey Bottle

Want to hear the 1991 Uncle Tupelo tune that jump-started the modern alt-country movement? The pre-Wilco/Son Volt trio explains it all.

The Rain Song

“Stairway to Heaven” may be the obvious choice of a Led Zeppelin inclusion, but “The Rain Song” is just as epic and even more beautiful.

God Save the Queen

It’s amazing how fresh and just plain “rockin'” The Sex Pistols’ infamously banned track comes across three decades later. Anti-establishmentism rarely proves this catchy.

Fight the Power

“It’s a start, a work of art.” Public Enemy’s 1989 rap triumph is still the best and most influential effort the genre has produced.

Cortez the Killer

Simplicity at its finest. Neil Young delivers a three-chord song about Europe’s exploitation of the New World that features as many layers of emotion as a classical symphony.

Joining a Fan Club

Jellyfish’s elaborate ode to glam rock sounds better than most of the acts it is emulating. I can’t recall a song that is just plain more fun to listen to.

War Pigs

Black Sabbath’s best-arranged (and best-recorded) track perfectly illustrates why it is among the most imitated bands in modern music. Heavy guitars and killer grooves – plus a surprisingly melodic-friendly Ozzy Osbourne – do the talking.

All Along the Watchtower

Jimi Hendrix turns Bob Dylan’s lyrical standout into a trippy showcase of guitar mastery that has never been equaled. Each solo seems better than the next.

Missed

For me it’s not a true list without a PJ Harvey song. While “Missed” is hardly her most popular effort, it reveals the Brit singer at her glorious, angriest best.

What’s Going On

The definition of soul music is embodied by Marvin Gaye’s deeply moving title track – a Vietnam War-inspired plea about the crumbling American dream.

Marquee Moon

The New York punk movement reaches its apex with Television’s 1977 debut, which includes a 10-minute title track of enormous grace and potency.

The Night They Drove Old Dixie Down

The 1978 documentary (and soundtrack) of “The Last Waltz” captures a live version of The Band’s folk opus with a brass ensemble complementing the sound. The Civil War-themed ditty is saturated with raw emotion.

Creep

Geeks and losers are granted a heavenly – though admittedly self-loathing – voice on Radiohead’s signature single.

Graceland

Paul Simon’s introspective songwriting is partnered with the mbaqanga music of South Africa on the experimental album “Graceland.” The title track stands as the musical antithesis of apartheid.

One

U2’s strongest song also endures as one of the great ballads of the last decade.

Shining Star

Earth, Wind & Fire is superb at fusing a batch of different styles into one cohesive mix, but the prolific group brings out the funk with this killer single. Here is the definition of a “rhythm section.”

Brass in Pocket

Music is frequently about attitude, and Chrissie Hynde supplies that to the nth degree on The Pretenders’ early marriage of pop commercialism and punk aggression.

Crossroads

Robert Johnson’s blues classic is filtered through the British Invasion on Cream’s live “Wheels of Fire” record. The barnburner represents the finest 4-minute stretch of Eric Clapton’s career.