Don't worry
If the time has arrived for you to read the Torah, first of all- relax. There is no need to feel pressure as that is the least constructive thing to do. There is no need to be embarrassed as everyone listening is your friend. Don't try to cram in some last minute review before you are called up (for your Aliya) as that can be very confusing
The best thing to do is to drink a glass of water and relax. That is what I did two years ago and it helps... So take a deep breadth and do the best job that you can!
Yours Chaim

A Tip for the Bar-Mitzvah boy
If you are getting an Aliya to the Torah, follow the Torah reading very closely and be aware of what is being read so that when it comes time for your Aliyah you will be prepared and you will not be surprised or nervous.
It is also a good idea to get up a short while before being called to the Torah so that you will not have to hurry and feel uncomfortable.
M.S. Yifrach

I would recommend practicing reading the first and then the fifth "Aliya" to the Torah and then if you are successful and you know it well, you can go on to the second and third etc...

Learning the Torah Reading
The first thing that you should do is divide the section (of the Torah reading) that you need to learn, and then it will be easier.

The most important thing is not to get nervous!

Catering for Shabbat Bar-Mitzvah
From experience (I can tell you) ,the caterers bring more portions than you have ordered. For that reason, on a Shabbat Bar-Mitzvah, it is a good idea to order less(e.g. 35 portions to feed 50 guests). This way you can avoid having a lot of leftover food after Shabbat, and also you can save money.

Highly recommended
It is very worthwhile to learn (for your Bar-Mitzvah) from this website.

Learning for a Bar-Mitzvah
It is a good idea to begin learning (for your Bar-Mitzvah) at least one year before the date, because if you don't you will be stressed and then it will be difficult to learn! ( from personal experience)

a Bar-Mitzvah is a once in a lifetime experiece. Yes, you get to see all the aunts, uncles, grandfathers, grandmothers, and the whole family. For that reason it is worth doing your best.

Reading the Torah
It is always a good idea to practice the Torah reading with a teacher or Rabbi. The tip is that it is also a good idea to proceed ahead on your own (beyond what you have learned with you teacher).