I am with a shared hosting and I can not access the logs.
Doesn't matter if it's shared hosting, the error logs are still domain or directory specific. I recommend changing hosts as without review of error logs it's impossible to debug the issue without many hours or days of trial and error.
How much should I put the memory limit and upload limit? I can ask this to my hosting service
Your limits are fine. If you're changing them with PHP.ini though then php.ini needs to be in Joomla root and root/administrator/.