ME1 isn't all that difficult, although I think it's harder than ME2.
Simple rule to follow: never kill anything while inside the Mako. Hop out and kill the enemy, no matter how big or bad it is (thresher maws included). The XP difference is enormous.
And, yes, the most important upgrade to have is something that boosts your shields. I forget what they're called, but use the upgrades that are +80, +100, etc. for your shields.
Other than that, most of the weapon and armor items you'll need will be found in the field, there's little need to buy it at a store (although the Normandy's supplier can get you ahead of the curve from time to time). The exceptions can be that sometimes it's hard to find good armor for your alien party members, so if you find something in a store that's better than what they have, go ahead and buy it. Money isn't really a problem in the game, you get so much junk you can easily sell it all and get up to the max number of credits really quickly.
As for playing the game, obviously you'll start with Eden Prime and go to the Citadel. Afterwards, I recommend tackling one of the mainline worlds that open and then do a bunch of sidequests. By doing that, and never killing anything inside the Mako, you'll be powerful enough to where the game will not overwhelm you.
**For the record, I played the game as a soldier and my party was almost exclusively Ashley and Tali. No biotics power in the party basically ever. I rarely used anything other than the pistol, either.
ME2 is entirely different, it's much more of a shooter than a RPG as far as combat, experience, and levelling goes. And it's easy.
