My answers to a buch of these questions...YMMV...
3. Where does the Imperium space/ control begin end?
12. Why don't the cargo/passenger prices change with tech level,distance, etc?
The answers to these are dependant on which version of Traveller you are using and which day of the week it is...
If you look around on CotI and the TML archives you should find these types of topics cropping up on a regular basis.
5. What is the difference between Ct, T20, New Era, Gurps etc? And can I use the deck plans/ships between them?
You may get various answers regarding the first part of this question, but this is my take on it.
The difference between the various versions of the game boils down to game mechanics and point in time in the traveller universe. CT, MT, TNE, T4, GT and T20 are all Traveller. Each version has different advantages and disadvantages and each of them have a potential wealth of material that can be used with any of the versions. At their heart they are all a game about a group of characters journeying in a science fiction environment. They each use a different (in some cases only slightly) set of game mechanics. Between the differing game mechanics and the default slant placed on the feel of the setting you can come up with just about any type of sci-fi game you want (which IIRC is one of the original intents behind the game).
Regarding the second part of your question...it depends. How much of a gearhead or perfectionist are you? Because the ship design and play mechanics vary between the different versions of the game, some ships that are common between them can end up with significantly different mixes of components taking up space within the ship. As a result, the deckplans often don't match up very well. On the other hand, if you like the deckplan and aren't concerned with it being an exact match to the ship's stats, go for it!
7. What are the abbrevations mean?
Depends on which ones you are asking about...
9. What book other the T20 handbook do I need to play?
Officially? A core rulebook from the d20 gamesystem. Unofficially? If you don't know d20 you'll probably want some book that has the full rules or the one of the d20 SRDs (System Reference Documents) which contain most of the d20 rules and are available on the web. Beyond that, there are many many many Traveller and Traveller-esque gaming books out there that could be recommended depending on your gaming style and what you are going to do with your campaign.
11. When can I take 10/ take 20?
14. How do I adjust CR from other monster books?
15. What are AOO(attacks of oppurnity)? And when do I use them? How many do I get per round?
23. What do I do when my character goes over 20th level.
These are all questions that can generally be answered by having access to a core d20 rulebook such as the Players Handbook or by looking through the SRD. I find adjusting CRs to be a bit of an arcane process that is highly dependant on the characters you are using so you may not find a satisfactory answer for that one.
16. Where are the free down loads?
22. Is there any free character generator or sheets?
I think you answered this question already...also, check the rest of the internet as well...there are a bunch of really cool traveller websites out there. Many of them have a lot of really useful stuff to grab and use. The Starport link at the top of the CotI forums is good starting point to get to some of these sites.
17. I have an old adventure from an earlier version, how do I convert it?
I would say wing it. Don't worry too much about any close conversions between versions. Read the adventure several times over and then jot down some notes on how to achieve the same effect for any given encounter using your ruleset.
18. I don't like this rule, can I drop it?
Not unless you like the rules police coming after you!

Just kidding...change what you want.
I would suggest that if you change a rule and then discuss your game or the way your game works someplace like TML or CotI that you may want mention the rule change if it has an impact on the discussion. By indicating that you are talking about an IMTU (In My Traveller Universe) rule change you'll be less likely to end up butting heads with some of the folks that REALLY know (and love) their particular set of rules.
21. I have old character what do I do to convert it to t20?
Wave a magic wand in the character's general direction and cross your fingers...
Seriously, I've found that converting characters to T20 from other Traveller rulesets is more a case of building the character anew based on what the previous character was capable of.