In addition to what chrono01 and Ala Alba said, the hardest chapter in the game is chapter 6 in terms of difficulty, but the game gives you enough help to let you carry on until then in chapters 5 and 7.
In terms of difficulty, prologue is the easiest, chapter 1 and 2 are around the same level (beginner level), chapter 3 ups it slightly, chapter 4 is intermediate level, chapter 5 dips into hard halfway through, chapter 6 is hardest (second to epilogue), chapter 7 is slightly harder chapter 5.
The game offers in game support to help you in case your party isn't levelled enough, and by the time you reach end game, they give you enough help to be prepared.
Which servant is the best I can get on my first free roll?
The first free roll that the game forces you through:
Hercules, Emiya, Elizabeth Bathory and Marie Antoinette are all the best top tier 4* guaranteed servants in the first mandatory roll.
Martha, Tamamo Cat and D'eon have their uses as well, so they are decent enough to use if you get them.
The other roll that you get from the free Quartz they will be giving out at launch:
Waver for easy mode, or any other 5* available at launch for an easier time in general. If it is the same as the rooster at the start of the Japanese game, then either Altera or Vlad are good. Jeanne is also great for defenses purposes.
Alternatively, complete the prologue chapter after the tutorial, save up the gift quartz, and roll for Saber Alter in the story gatcha for the best 4* Saber in the game, or roll in the banner for Nero Festival for the other best 4* Saber in the game.
Also, 3 stars, 2 stars and 1 stars are going to always be viable and important for your team throughout the story.
The tutorial roll is guaranteed to have a 4* servant, and cannot give you a 5* servant. In that roll, generally the best is Heracles (Emiya is also eventually really good).
However, the Facebook campaign is giving us a bunch of free quartz to roll with, and the regular rolls have a 1% of a 5* servant. There will probably be only 5 5* servants to start with, and the best 5* servant you can roll out of those five (and probably for about 10 months after launch) is Zhuge Liang (though he's more of a late bloomer).
To be honest, I would recommend that people focus on 4* when rerolling for the story gatcha at first rather than Waver. An NP2 Heracles or Saber Alter/Nero is more useful early game than Waver who needs a lot of investment to make him really great.