Memorizing, that's something you can learn. First of all, don't worry too much before you have started on the piece. You don't know, maybe you'll memorize very easily.
There are many "tricks" to memorize. I'll pick some out for you.
Here's the first link:
I suggest you to read this part. You'll understand more about memorization.
And here are some links to other forums where memorization is discussed:
http://www.pianoworld.com/ubb/ubb/ultim ... 12266.html
http://www.pianoworld.com/ubb/ubb/ultim ... tml#000000
http://www.pianoworld.com/ubb/cgi-bin/u ... 11894.html
http://www.pianoworld.com/ubb/cgi-bin/u ... 10086.html
http://www.pianoworld.com/ubb/cgi-bin/u ... 10111.html
http://www.pianoworld.com/ubb/cgi-bin/u ... /9924.html
Maybe they'll help you. You can even register there if you want help. They are usually very quick with responding (it's a huge forum).
As for finding an orchestra, I don't know. Maybe you can tell us a more about yourself, like how old you are, where you live, etc. If you are at a high school, mostly they have an orchestra. When you are at college/universiry, they usually have a student orchestra. When you are an adult, it's harder to find one. You should be able to find a local one, and ask if they want to play with you. Maybe they want a kind of audition. Of course you have to play well then. And if you can't find an orchestra, you can always use a second piano. Perhaps someone you know can play they orchestra part. Maybe your teacher? Do you have a teacher?
I hope this helped
. Good luck with learning.
EDIT: I agree with Robert, as a first concerto, Tchaikovsky is not a very good choice. I don't know about your level or repetoire, but it seems to me that starting with Tchaik 1 is very difficult.