Your local shop will probably sell the tools you need to take the crank apart. Definitely take the bike in, because one of the oddities of the Denali (and Wal-Mart bikes in general) is that the parts aren't always standard, and with cranks and bottom brackets there are several possibilities.

The bonus with your shifting adjustments is that after the experience you're gaining getting that bike to shift well, you'll be able to tune a higher quality shifter/derailleur pair in your sleep whenever you decide to upgrade.