I am concerned that I am not smart enough
No one is smart enough.
But if you mean, specifically, smart enough to
aspire for rationality
then I think the question is kinda backwards. "Am I too stupid to try to improve my thinking?" -- it's like "am I too sick to try to improve my health?" or "am I too weak to try to improve my strength?" or "am I too poor to try to get more money?".
Now, no doubt all those things are possible. If you really can't reason at all, maybe you'd be wasting your time trying to reason better. And there are such things as hospices, and maybe some people are so far in debt that nothing they do will get them out of poverty.
But those are unusual situations, and someone who is headed for a good result in a challenging subject at a good university is absolutely not in that sort of situation, and if the stuff on Less Wrong is too hard for you to understand the fault is probably in the material, not in you.
I find it difficult to multiply two 2-digit numbers in my head
A fine example of the kind of "easy" task human brains (even good ones) are shockingly bad at. I just attempted a randomly-chosen 2-digit multiplication in my head. I got the wrong answer. Am I just not very intelligent? Well, I represented the UK at two International Mathematical Olympiads, have a PhD in mathematics from the University of Cambridge, and have been gainfully employed as a mathematician in academia and industry for most of my career. So far as I can tell from online testing, my IQ is distinctly higher than the (already quite impressive) Less Wrong average. It is OK not to be very good at mental arithmetic.
(Having said which: If there were something important riding on it, I'd be more careful and I'm pretty sure I could do it reliably. I did a few more to check this and it looks like it's true. So I may well in fact be better at multiplying 2-digit numbers than you are. But the point is: this is not something you should expect to be easy, even if it seems like it should be. And the other point is: Even if you are, in some possibly-useful sense, less intelligent than you would like to be, that is not reason not to aspire to rationality. And the other other point is: It's clear that your intelligence is, at the very least, perfectly respectable.)
A few notes about the site mechanics
A few notes about the community
If English is not your first language, don't let that make you afraid to post or comment. You can get English help on Discussion- or Main-level posts by sending a PM to one of the following users (use the "send message" link on the upper right of their user page). Either put the text of the post in the PM, or just say that you'd like English help and you'll get a response with an email address.
* Normal_Anomaly
* Randaly
* shokwave
* Barry Cotter
A note for theists: you will find the Less Wrong community to be predominantly atheist, though not completely so, and most of us are genuinely respectful of religious people who keep the usual community norms. It's worth saying that we might think religion is off-topic in some places where you think it's on-topic, so be thoughtful about where and how you start explicitly talking about it; some of us are happy to talk about religion, some of us aren't interested. Bear in mind that many of us really, truly have given full consideration to theistic claims and found them to be false, so starting with the most common arguments is pretty likely just to annoy people. Anyhow, it's absolutely OK to mention that you're religious in your welcome post and to invite a discussion there.
A list of some posts that are pretty awesome
I recommend the major sequences to everybody, but I realize how daunting they look at first. So for purposes of immediate gratification, the following posts are particularly interesting/illuminating/provocative and don't require any previous reading:
More suggestions are welcome! Or just check out the top-rated posts from the history of Less Wrong. Most posts at +50 or more are well worth your time.
Welcome to Less Wrong, and we look forward to hearing from you throughout the site!
Once a post gets over 500 comments, the site stops showing them all by default. If this post has 500 comments and you have 20 karma, please do start the next welcome post; a new post is a good perennial way to encourage newcomers and lurkers to introduce themselves. (Step-by-step, foolproof instructions here; takes <180seconds.)
If there's anything I should add or update on this post (especially broken links), please send me a private message—I may not notice a comment on the post.
Finally, a big thank you to everyone that helped write this post via its predecessors!