Welcome to DTT, Adam!
You've already made the best possible first step: joining up here and asking advice! Next thing you should do is read. Read lots . Check for build threads based off of CM400's to get some inspiration. Check for service manual downloads in the Manuals section. Ask questions if you don't know and try not to get offended if your answer comes buried in sarcasm or dick head remarks (most folks here are great, but some can come off a bit over harsh). Remember the only stupid question is the one you ask over and over.
If your new-to-you bike isn't a runner, then fix it to rideable condition before cutting it up. If you live somewhere with a riding season then just get it on the road for this year! Winter is for building, and winter is almost done. Get the bike running, make plans and set a budget (these things get expensive, quickly!).
Finally, HAVE FUN WITH IT!