Hey there, I've been on pokerstrategy for a few months, switching between all of the different gametypes, unsure of which one to choose, but I think I have decided to stick with No Limit, since I've heard it's the most challenging, and so therefore the easiest for your opponents to make mistakes, and get you profit. That is, if I can make less mistakes than them. 
Question 1: What is your motivation for playing poker?
My main motivation is obviously going to be seeing how much profit I can make, if any, but also trying to have fun along the way. I don't like the feeling of other players outplaying me, so realising that the other players are better than me would also motivate me to improve.
Question 2: What are your weaknesses when playing poker?
I tend to trust too much in things like the starting hands chart, or playing hands just like in the articles, so need to get better at looking at every hand differently and not getting stuck with one way of playing a hand. Also when people make a lot of action against me I think I get too nervous and want to fold too easily, so I'm sure learning more about the game will help me feel more comfortable in those situations.
Question 3: What does it mean to play tight aggressive?
Tight - choosing only selected hands to play, those which give you an advantage EV wise over your opponents.
Aggressive - When you play a hand, you generally are the one who bets and raises, trying to control how it plays out.
Safer is to fold the hand than invest money where you already behind and might at the best be against strong draw or anything similar. But of course if you want to learn more about the game I would strongly advice to try using Hand Evaluation forums.