You start by punching a tree.
Now to be serious, for thaumcraft you'll definitely want to get some scribing tools so you can make a research table. Researching items via the table saves you from polluting the aura while doing research (the other method is via cauldron and can cause a ton of pollution).
For forestry you'll probably want to look up some videos and builds to help with all the stuff it adds. The farming stuff is fairly easy to setup, for example, but if you place it in the wrong spot you'll suddenly see it eat a hole in your house. Also, if you plan to work with bees you'll want to read up on them as they can be difficult to work with if you're new to them. That said, beekeeping can help a lot in the long term via all the things they're able to help you produce. Since you have some experience with Buildcraft you should be able to setup the Forestry machines with minimal challenge so it shouldn't be too hard to get in to.
As for something to work on, if you have railcraft in the Magic Pack (haven't checked for it :X) then work towards a boiler system. You should be able to use Peat Bogs to fuel a solid boiler and you can setup biofuel production for some liquid boilers. Max sized high pressure boilers are what I would call a staple for MJ production in your main base. While they take longer to heat up than low pressure boilers, high pressure boilers produce twice the energy for the same amount of space, making them more space efficient while keeping the same fuel efficiency at max temperature. Each max sized high pressure boiler can produce steam equivalent to 144 MJ/t.
From there you can replace any low tier engines and connect them to your boiler in a variety of ways. You can either have golden waterproof pipes piping steam to an industrial steam engine that then powers a machine, or you could invest into some energy conduits and attach your steam engines directly to the boiler. The energy conduits act much like the conductive piping from BC, but are better at their job. you can also use Energy Cells to store any excess energy in your system for later use, such as to power quarries.
Basically, there's a ton of stuff you can work on and you should set some goals for yourself. My goal, once the Ultimate pack comes out, is to use TC3 levitators to send me above the clouds into where I'm going to build my massive sky fortress that will be the hub of my power creation and magic research, as well as a transport area to other dimensions. It's gonna be 5x5 chunks for most of the working area, with a roof that's slghtly bigger than 7x7 chunks. The roof will be an 8 pointed star with a created aura node on each point, and a larger node in the center of the area on the roof. I'll have all of this connected to the ground which will have my farming areas via tubing and energy conduits to keep the energy going. I don't know about you, but I think having a flying fortress in the sky sounds prety damn sweet.
Edit: With thaumcraft, Azanor has stated only about 25% of the planned content is in the game currently. There are several things in the mod which are useful, such as being able to transmute metals of one type into another, levitators that slow your fall or lift you up (and if stacked on top of each other increase this effect) and golems which act similar to pipes and can farm (not sure if the golems work with IC2 crops, if they do that could make them better than forestry farms). It also adds various fun items that have some functionaility, like the hungry chest which "eats" items dropped onto it (for use with the arcane furnace) and the brain in a jar, which draws nearby experience orbs into it and you can get them out by hitting it....but it stares at you. There are also weapons such as the fire and lightning wand, as well as an excavation wand and a wand that lets you replace blocks with other blocks in your inventory (such as replacing a large amount of obsidian with dirt).
there's no set way, there's no guide, go dig and you'll expand from there, once you have ore.