Which one is better? Dual Pistols or a Rifle?
That depends on how you play it, the stats you go for, the skills and traits you use, etc.
I would say: try them both from time to time and use whichever one you enjoy more. I was a fan of pistols for 50-some levels and then recently switched to rifle and there’s no reason I might not switch again sometime.
General rule of thumb: if you use pistols, go for Condition Damage rather than Power. If you use Rifles, go for Power rather than Condition Damage. But at low levels it doesn’t much matter.
What are your favorite skills to unlock? Like the Flame Turret, Healing Kit, etc.
You’ll want to unlock everything, eventually. You’ll have plenty of skill points to throw at it. The Engineer is a versatile class and I frequently change what I have on my bar for different situations.
I recommend getting Healing Kit and Healing Turret both — the turret is better I find, but the kit is handy for faster travel.
And finally, what do you think is the best race for a Engineer?
There are a couple recent threads on this. It comes down to personal choice. Mine’s an Asura. I can’t stand playing Charr because of the run animation and I hate their capital city and one of their early zones.
I’m thinking about restarting my Engineer, and moving it to be Charr for it’s race.
If you have an unused slot, and you haven’t played a Charr yet, start a new character there instead. See which one you prefer. If you do switch, be sure to use the bank (conveniently accessible from all crafting stations) to transfer your coins and anything that’s not soulbound to your new character.
Also, if you’re feeling so generous, I’m very new to the game, and wondering what the best way to level is, but this isn’t the place for that, so! 
My general advice:
— gather everything you can.
— craft as a way of earning XP and providing yourself with basic gear, but be aware of when it’s cheaper to buy stuff from the auction house than it is to acquire the materials to build it yourself.
— hit every World Completion spot you can get to.
— keep your gear reasonably up to date, but don’t overspend on rare gear.
— try to be a few levels ahead of the personal story missions. Visit other zones in your level range and below. Some story missions can be surprisingly nasty.
— finish daily achievements if you can; you get extra stuff.
— don’t attack Champions alone. With rare exceptions, they will murder you before you scratch their health bars. (Once you get used to dodging, some can be soloed with caution — like the big worm under the Durmand Priory.)
For engineers:
— I recommend putting your first 10 trait points (which you get starting at level 10) into Tools. This will let you choose Speedy Kits, which gives you a big speed boost whenever you switch to a kit. (Since you can switch to Healing Kit for free with no cooldown, or other kits with 1 second cooldown, you can run fast forever outside of combat.) And then for combat, you can switch that trait to Static Discharge, which is a nice bit of damage every time you use a toolbelt skill. Rifle Turret gives you a short-cooldown toolbelt skill that’s perfect for it — much better than the actual turret itself.
— Like I said, try everything :P Don’t get discouraged; try switching your gear, skills, traits, etc. until you find what works best for you.