The average Social Security payment to a retired worker currently sits around $1,234 per month -- or about $14,808 per year. For comparison, a full-time minimum wage job would currently pay $7.25 per hour -- or around $14,500 per year. Or in other words, the average Social Security check would get you right around a minimum-wage lifestyle.

Unfortunately, that's the good news about Social Security. The bad news is that even that meager payout is on track to be slashed. According to the 2012 Social Security Trustees' Report, the program's Trust Fund is expected to run out of money in 2033, cutting benefits by about a quarter. Or in other words, that $1,234 monthly check is on track to become an inflation-adjusted $925.50, within the next 21 years.

That's Social Security's promise
If you want to live your golden years on a less-than-minimum-wage lifestyle, go ahead and rely exclusively on Social Security. The program's trustees have been painfully clear about what you can really expect, both today and in the future once the Trust Fund empties. It isn't pretty. It isn't fun. But it's the future you will face if you're planning on Social Security as the sole source of your retirement income.

Fortunately, if there's a bright side to the problems Social Security is facing, it's that its Trust Fund's collapse is happening both fairly slowly and out in public. That combination means that you can see it coming and have about two decades to prepare for its arrival. Still -- that time doesn't give you clearance to ignore the problem. In the world of investing, 20 years is a decent amount of time to build a nest egg, but if you don't start now, the opportunity starts to evaporate quickly.

Use your time well
Even if all you're trying to do is cover the gap left behind when the Trust Fund evaporates, you'll still need a fairly large chunk of change. Using the 4% rule as a guide, you'll need around $92,550 -- nearly $100,000 -- to cover the typical expected Social Security shortfall. And remember -- that's just to get you back up to the minimum-wage lifestyle expected on average from today's Social Security.

If you'd like a lifestyle more in line with what you've been earning while working, you'll want to sock away more. Using that same 4% rule, for every $4,000 in annual income you want to replace, you'll need around $100,000 saved. The chart below shows the amount you'll have to put away each month for every $100,000 you want to sock away, based on different potential return rates and times:

Years to Go
10% Annual Return
8% Annual Return
6% Annual Return
4% Annual Return
20 $132 $170 $216 $273
18 $167 $208 $258 $317
16 $213 $258 $311 $373
14 $275 $325 $381 $445
12 $362 $416 $476 $542
10 $488 $547 $610 $679
8 $684 $747 $814 $886
6 $1,019 $1,087 $1,157 $1,231

Source: Author's calculations.

As you can see, it gets progressively tougher the longer you wait. And if you think that it is difficult to come up with $170 per month to invest now, imagine how much tougher it will be to come up with near $1,100 per month in 14 years. And all that's just to be able to cover $4,000 in annual income, some 20 years from now.

It's time to start investing
If you haven't been much of an investor before, getting started may well be the toughest part. But take heart -- even if you're worried that the results won't work out as you'd expect them to, know that even lousy investing still beats not investing at all. What's most important is to have a reasonable strategy and stick with it -- and let the time and the magic of compounding work to your advantage.

Your reasonable strategy may well be to buy an index fund, like the SPDRs (NYSE: SPY) that track the S&P 500. That would give you instant diversification and fairly broad market coverage, within the U.S. stock market. If going global is more your style, Vanguard's Total World Stock Index (NYSE: VT) ETF can help you cover the world, with one-stop investing. Of course, if going from "nothing" to "all stock" doesn't sit well with your psyche, adding broad bond exposure like Vanguard's Total Bond Market (NYSE: BND) ETF could help temper those concerns.

Any way you approach it, though, making a commitment to yourself to start consistently and regularly investing is a critical first step. With a mere 20 years or so left until Social Security's trust fund is expected to empty, the time to start taking action is now. Wait much longer, and all you'll be left with is the minimum-wage (or less) lifestyle that Social Security is offering its typical retiree.