Welcome to FTB. I played Minecraft for a while before knowing that there were mods, so I do not know if mods would have overwhelmed me. But I trust the wisdom of my fellow forum members. Hope you have a fun time exploring vanilla before comeing back to FTB.
As for finding diamonds, here are a few tips.
1) You can get your coordinates by pressing the f3 key on the top of the keyboard. x and z are north and east (forget which is which though) and y is up. Diamonds form around y=10 to y=17.
2) It wastes a lot of time mining every block, instead build a main tunnel and make branches off of it. Make all tunnels 1 wide and 2 tall. Place the brances with three blocks in between. Layer the floors so the tunnels are staggered. Like so
Top View (0=dig,x=don't dig, L=ladders to travel between floors)
floor 1 (feet @ y=9)
xxx0xxx0xxx0
xxx0xxx0xxx0
L0000000000
xxx0xxx0xxx0
xxx0xxx0xxx0
floor 2 (feet @ y=12)
x0xxx0xxx0xx
x0xxx0xxx0xx
L0000000000
x0xxx0xxx0xx
x0xxx0xxx0xx
floor 3 (feet @ y=15)
xxx0xxx0xxx0
xxx0xxx0xxx0
L0000000000
xxx0xxx0xxx0
xxx0xxx0xxx0
This pattern will allow you to see more than 66% of the blocks in the area you mine while only requiring you to mine 16% of the blocks, and every block that you do not see is touching 3 blocks that you do see. Diamonds generally form in veins. A 2-vein with one block hidden has a 50% chance that the other block is visible. A 3-vein with one block hidden has a 1-(3/6)(2/5)=80% chance of having a visible block.
To sum up, this pattern finds almost all veins of diamonds that are 3 or larger, 85% of the 2-veins, and 66% of all singleton diamonds and requires a low amount of work.