Mass Effect 2 should really have more than one guide because there are things that I would do differently depending on who I was planning on romancing in this game, if anyone at all. I think I will explain this as I go along, but there are areas where there are multiple orders of missions available to you.

I have tried to keep this as spoiler free as possible. I have also split the game into three acts, as in my opinion it's the only game of the trilogy that fits this.

Some tips:

Don't buy fish right away: Speak to Kelly as often as you can and be a bit flirty even if you have no intentions of pursuing this. Eventually you will have a dialogue option along the lines of "You do care!". Invite Kelly for dinner. When you speak to her afterwards, she'll offer to feed the fish for you! At that point, buy all the fish you want! (This also has an impact on the next game.)

Kelly is also good for telling you when the loyalty missions unlock for your team mates so check in with her often. Joker has many quips after every recruitment and a lot of fun dialogue in what I would refer to as act one.

Scanning planets: You will have enough elements if you scan the planets that are marked as Rich until they switch to Moderate. Sometimes they will also unlock something you can land at. These never have unique dialogue so, if given the opportunity, take whoever you want.

When you end up back in the Normandy speak to everyone all the time especially if you have just recruited someone. There's so much more dialogue than in ME1 and oh boy it's wonderful. If they have an option for [NORMANDY UPGRADES] ask them. Should you have the resources you will be able to upgrade straight away but if you don't you can upgrade later on in the tech lab.

If you're looking for character builds, check out this reddit post. You no longer have to worry about putting points in Charm or Intimidate this time; your actions in game give you Paragon or Renegade points. Very early on you will learn about the addition of quick time events too. Nothing actively hinges on these in this game.