Starts one mod at a time. Usually each mod (except addons) works just fine alone. The fun in modpack is combining them but obviously you aren't there yet.
Here's some mods that you can work on early that are usually used by every modpack.
-Tinker's Construct : You usually get the book that helps you with this mod too. You can move on if you have a smeltery.
-Thermal Expansion 3 : Start with making a steam dynamo, pulverizer, redstone furnace, induction smelter and some conduits, Make sure to make the crescent hammer to break them. If you have TE2 you don't have the easy to do conduits but that's fine, the wrench replaces the hammer. You're set if you can make a magma crucible along with a fluid transposer.
-Thaumcraft 4.1 : make an iron capped wooden wand and a bookshelf. right click the bookshelf with the wand and you get a book that explains everything. You should make a thaumometer by then and scan stuff. This will take a while since you can't scan everything at first so you should start this mod early.
-MineFactory Reloaded : adds machines that works just fine with TE3 but you might also need fluiducts and itemducts around them.
-Ars Magica 2 : make a book and a frame, place the frame next to a pool of liquid essence and the book in it. The book then tells you everything you need to know. I suggest you get dig with projectile first.
-Buildcraft : Usually used for the Quarry. Look it up it's really usefull.
Then you have mods that makes the world look better and adds a lot of options for builders.
-Biomes O' Plenty : Adds alot of biomes (duh)
-Bibliocraft : Adds furnitures
-Chisel : 1 stick and 1 iron bar in a diagonal pattern. You can now retexture those blocks you have in big quantity like dirt, cobble, stone, sandstone, glass, etc. Also adds Marble and Limestone.
-Forge Microblocks : You can reshape blocks into smaller versions and add some small details to your builds. Make a saw and craft it with a block you want to cut. you can make panels, small blocks and even hollow them by making a O shape on a crafting table.
There are a lot more but those can be learned early on.
