I'm going to be playing it on the Casual setting so I can try to obtain as much as I can before going on to Hardcore and what not. I also want to try and max out as much as I can for my character and teammates. Which type should I start off with? Also, do I let it automatically level up or do I manually level up my characters? Which weapons are best and any Combos that I can start off with?

Should I use Wrex, Ashley or the alien dude?

It doesn't much matter which character you side with you (especially on Casual). Try using characters that compliment your abilities or just use your favorites. I always have Wrex with me because he's awesome.

Wrex and Liara sound good?

I wanted to stick with Ashley since her attack powers are great, but I do want to get some achievements. I'm also thinking about sticking with Ashley and Liara.

What class did you choose? If you're a tech specialist, Ashley and Liara would be a good team.

Soldier actually. I'm about to head out for Liara. Right now I have Wrex, the alien dude and the alien girl. Can't remember her name. What should I go with?

If you're a soldier and want people that compliment your abilities, I'd go with Talli (if you have her, if not take Garrus) and Kaiden.

Is Kaiden the Human?

For one of the Ally Achievement, can I use Ashley and other? Main reason I ask about Ashley, I want to beat it the first time without any problems. If I do use her, would I get an Achievement with her? I really want to pair up someone decent with her.

Kaiden is the human. As for the ally achievements, I don't really know. I've completed the main and side missions with one of the allies you supposedly get an achievement for more than once and got nothing.

Hmm. So it seems a bit more difficult then. Did you switch you team mates often or did you stick with them for the most part of the game?

I stuck with them.

Damn. So there must be some extra shit to it. I'm mainly restarting so I can go for the achievements. I love the game and it's fun, so don't get me wrong. But I'm going for achievements.

I've done the same thing DMD said (sticking with the allies from the beginning and doing most side quests) and never got the achievement. I got the "Completionist" achievement but not the ally one.