Watching youtube videos is really helpful. Just be careful that you either find one that is up to date or watch an old one while being aware of potential changes.
Also, what you do depends on which mods you
want to learn. There are three branches, really: Tech, Magic, and Other (with other being divided into Exploration, Utilities, and Miscellaneous).
I would recommend
Direwolf20 as a starting point. I advise you to watch his
Let's Play series (I've linked to the most recent series, which is still actively being played). The videos might be a little long, but there's a lot of information and it's (mostly) done in tutorial format. He specializes in tech, but he also covers magic fairly well and uses stuff from "Other" fairly often. His modpack is available on the FTB Launcher under public packs; it's a kitchen sink pack with a little bit of everything. If you get bored watching his Let's Play, you can switch over to his Forgecraft series, which is just him playing MP. It's a lot more interesting, but he doesn't hold back in terms of information flow and he cuts out whenever he needs to, so it isn't as good for tutorial use.
Modded MC is infinitely better than vanilla. Nobody here will tell you differently.
//Shameless Self Promotion
I have made a modpack called
Colony which is specifically meant to be easy, geared more towards sandbox play (not creative; survival sandbox - the challenge is in BUILDING, not GATHERING). It has no magic mods, as I enforce strict theming rules on everything I make.
You'll get up and running much quicker than usual - that should be helpful for a beginner. You wouldn't want your experience to just be one giant mining session.
If you do decide to both watch DW20 AND play Colony, be aware that there might be a *few* mods that are not shared between both. Due to the "kitchen sink" aspect of DW20's pack/playthrough, most of what's missing from my pack are magic mods, and the extra stuff you'll get with Colony are mods that don't have a large name for themselves, stuff few people use or know about, but which is really cool - hipster mods, if you will.