I'm able to hit all low noted but I'm not able to get sound out of High Notes.
What could be the problem?
Also what reeds shuld I get and What # ?

Need more info, like how long have you played, what horn, what reeds exactly, picture of your embouchure (j/k). Even still you'd be better off finding a live teacher to take advice from, as there are a large number of variables involved. Good luck.

I played for about 4 month.
I use the reed thet came with my clarinet. (I think its 2 1/2)
my Clarinet is Simba.

at what note do you start having problems?

You may want to try a stiffer reed. I recommend Vandoren, but keep in mind they tend to be a bit stiffer than the same number of reeds such as rico, so stay at 2.5 if you switch brands.

If you've been playing on the same reed for four months, you may be having problems with the reed being blown out. Eventually, even if it isn't chipped or warped, a reed will begin to lose it's good sound, soften, and in general get bad. Try a new reed and see what happens.

When trying to play high notes with a softer reed, scoot the reed up on the mouthpiece a tiny bit. I'm not sure why, but this helps. This was what my band teacher told us in 5th grade as we were extending our ranges.

SUPPORT is vitally important with high notes. use LOTS of air, and keep the corners of your mouth firm.

Above all- Practice Practice Practice! Good Luck!

at what note do you start having problems?

I think Its G5
And some times a little above or bellow G5.

The reason the reed is moved up on the mouthpiece a bit is to give the mouthpiece tip a stiffer part of the reed to vibrate. This way it has a little more resistance than a completely worn and thinned reed. I would definitely suggest new reeds to the original poster! I don't think 4 months is a good length of time on a reed. In the beginning I wouldn't go more than 3 or 4 WEEKS...I also buy reeds harder than I play, and shave them down a bit...but wouldn't recommend this to someone that hasn't been playing very long. Vandoren reeds might be a bit stiff for you. Rico Royal is a good reed, along with Mitchell Lurie. I might suggest a 2-2.5 inRico...or a 2.5-3 in Mitchell Lurie. You do want some substance in your reed for the high notes. Otherwise, they will crack on you.

-Dennis