Okay…numbah one…I got a little " grammar" problem with your question…Do you mean " How do I start planning at 40 ? " or do you mean " How do I plan to retire at 40?"
If you want to retire at 40 you can’t use IRA’s etc…it’s just outright save and invest every penny you can find between now and then…give up all luxuries with that " 40 goal" in mind… but bear in mind…even if you make great investments between now and then, you’ll be giving a lot to the politicians to " play" with ( TAXES)
Now…on the other hand…if you ARE 40 and you want a plan…it’s almost the same…save and invest, save and invest…BUT in your case you can make those investments in a ROTH IRA… make THOSE wisely and it’s ALL YOURS…when you’re 60…
At forty, you’re playing " catch-up" so make some of your investments in the sectors that are currently "high-flyers"…
hoping for returns in the high teens, low 20’s…forget about bonds and safety for about 7 or 8 years…. a good international fund could be part of your catch-up…and maybe some ETFs in agriculture, energy… MOO, DBA, USO OIH.
BUT, those are things you have to keep abreast of…and be ready to move to what gets " hot" maybe this Christmas season…certain retailers..maybe " tech" in general…REITs could even be making a come-back by then.