1. Should I choose a one topic like current affairs or only bitcoin or only altcoins discussion or should I post everything here.
The main purpose of the Bitcointalk forum is to discuss Bitcoin or other crypto related discussions, you don't have to use 1-2 discussion boards only, You other boards are better to discuss, such as: gambling, games, Economy, services, trading, Meta, and many others, which essentially addresses your crypto expertise to add knowledge to other members.
2. Should the post has old information?
old information if it is needed is not a problem, but the latest information is better.
3. Should I post only links which define my whole post.?
the core and purpose of the post is better explanation, links if needed, the important thing is that people understand the essence of the topic or information you provide.
4. Should I reply to every one who reply to my post? ( 4th number add now because I think this is compulsory question)
if the person asks and needs more information from you and doesn't understand it, you are obliged to explain the gist of the problem, no need to answer all the points you hug.