You can install many apps - what do you mean " not many apps " ?? Your Tab has 2 GB space to install apps - this means you can install up to 1000 apps or more !!

http://forum.xda-developers.com/showthread.php?t=933400 : this is master guide on various things you can do with your TAB

Please refer also post # 2 and # 3 for some specific answers

Your question is too general. Anyway here is a quick rundown :

Apps - install download and install directly from Market. You need to have a gmail account for this.
If you have access to apps APK files, just copy it over to your card, tap on the said apk and it will auto install
If you wish to access paid apps market - download install MarketAccess or MarketEnavler to simulate foreign telco - refer post # 2 Q11 for steps

You can upgrade your tab :

1. Change kernel
2. Change default RFS file system to EXT4
3. Change roms

Refer post # 3 on Rom Flashing , FAQ for quick guides or refer to the XDA link I have given above

Do read first before attempting these stuffs. It is not difficult

Is your TAB rooted ? How did you installed superuser as this is only done through rooting not be separate installation

Is there a app Supersuser in menu - it will be there after root  ?? You don't root by just installing an app to create superuser . Can you point me the link where you download so that we know what you are actually installing

Easiest way to determine:
I believe if you see an icon superuser as one of the app, it meant that you are rooted.  To double confirm, download Titanium backup and install it to see if it can backup.  If it does, you are rooted :-).

Alternatively, download z4root may be another choice :-)

I don't think it will work. I read the developer comments - seems to be is more of a patch to existing superuser app Why don't you use the usual guarantee methods of rooting - z4root or superoneclick ?