This trophy should come somewhat naturally. A million credits is not as daunting a number as it may seem.
While on a planet with the Mako, take the time to complete each planet. Search for minerals (probably the biggest source of income), look for anamolies and other markers. Complete side missions and fight enemies. You will also obtain credits from enemies after defeating them. Many quest givers will give you credits once their quests have been completed. You may also be able to persuade getting more credits from certain NPCs by using your Paragon/Renegade options.
Finally, selling stuff comes into play as another huge money maker. If you've been putting your points into Renegade, you will get bonuses which allow items to be sold at a higher rate than normal. This can help if you need it (but you shouldn't). Just sell any duplicates of items you have and items you won't use. If you've been taking the time to complete the game, you'll get this through normal story progression easily.