The Bioware blog recently posted about this information, which I'll summarize here.
Mechanics
Earning reputation in Mass Effect 3 is a bit different from the first two games. Rather than earning only Paragon and Renegade points, you can also earn Neutral reputation, so there are 3 types of rep in ME3.
This means you can now mix being Paragon/Renegade without worrying about getting locked out of the more difficult Charm/Intimidate options. When you earn Paragon/Renegade rep, it'll show up as Blue/Red, whereas if you earn Neutral rep, it'll increase your reputation bar's size as a whole while retaining the same blue/red ratio.
Click through for the larger version.
Your total reputation (blue/red/neutral) will determine whether or not you'll be able to pass a Charm/Intimidate block. The lines on the bar are "checkpoints" which serve as gates for more difficult dialogue options. I marked them with the thin lines of red, in case it was difficult to see (this is not my Shep, this is the photo from the blog post).
As per usual, you get rep from completing missions and making dialogue choices. For missions with no Paragon/Renegade choice but still have a large impact, you will get Neutral rep.
Story Impact
In contrast to older Bioware games, you can never actually be the villain in ME3, though this should be rather clear from how they handled the first 2 games. No matter what, you will be fighting to save the galaxy. The manner in which you do it will be what differentiates a Paragon from a Renegade. Also, bullying people into giving you more money or discounts won't be the crux of what makes a Renegade decision in this game. Who needs credits if there's nowhere to spend them because civilized life has been destroyed?
Reputation gain will be based on when you have to make a decision in the game. Here are a few examples of what kind of rep you would get, taken from the blog:
- Confronted on the Citadel by a desperate refugee with a gun, you give her some credits and help her find a place to sleep. (Paragon)
- As a human colony falls to Reaper forces, you order down an orbital strike, brutally killing thousands of colonists to prevent the Reapers from turning them into husks. (Renegade)
- You land at a turian fuel depot taken by Reaper forces and clear it out, enabling allied forces to keep fighting. (General Reputation)
TL;DR
BioWare has finally made a single-player game that lets you make morally grey choices without nerfing yourself, gameplay-wise. Rejoice!
In ME1 If you choose to Sacrifice the Council, you lose the Destiny Ascension worth 70 TMS, but 4 Alliance fleets don't take losses totalling 100 TMS, netting you 30 TMS for the Renegade action.
EDIT: You net 25 more points by saving the Council, assuming you save the Salairan Councilor in ME3.
- Save the Council: 120 points
- The Destiny Ascension: 70 points
- Alliance First Fleet: -25 points for ships lost during the rescue
- Alliance Third Fleet: -25 points for ships lost during the rescue
- Alliance Fifth Fleet: -25 points for ships lost during the rescue
- Allows you to save the original Salarian Councilor in ME3
- Gives you the Salarian Third Fleet: 125 points
- Don’t save the Council: 95 points
- Admiral Mikhailovich: 25 points
- Allows you to save the replacement Salarian Councilor in ME3
- Gives you the STG Task Force: 70 points
During the Grissom Academy mission choosing to use the students as Artillery instead of Support gives you 75 instead of 50 TMS, for a gain of 25.
After the Quarian/Geth war, if the Quarians are still alive, Tali and Xen argue about whether to prioritize ships for rescuing survivors or salvaging Reaper tech. The latter grants you Advanced AI Relays for 45 TMS, and seems like a "Renegade" option.
Just about everything else I can think of can be resolved with charm/intimidate or is better off with the paragon choice.
Best Answer
It seems that you need to be at the third rank of the reputation to be able to convince Din Korlak to send his fleet to save the Volus' One.
This will cause Turian Sixth Fleet to suffer -10 points, but you'll get voolus Bombing Fleet which value is 75 points. Unless you Zaed is alive, where there will be no penalty.
Source: List of maximizing War Assets thread on Bioware forum