Happy to help, where I can.
Let me see if I can give you the steps to add a partition.
The first step, I think you have done, and that is to run a partitioning tool to create a partition on un-allocated space on the drive.
Next you would format that new partition with the file system you want. Linux supports many different file systems. Since you are using, or are going to use Ubuntu, select ext4 file system. That is the default in Ubuntu, and many other distros. The command would look like this:
This command as root, would format the partition sda5. You do need to be careful with this command. You need to be sure what partition ( like sda5 ) you are going to format. Once you format it, any data is gone.
Once the file system is created, you need to create a mount point. A mount point is just an empty directory. There are usually some already created on most systems. /mnt and /media are just two. You can use them, however, I suggest you do not use /media. It is there for 'automounting' some devices, like flash drives.
The mount point is really your choice, and a lot depends how the data on that partition will be used, and what users will use it. I have a multi-user system and use /mnt. So, assuming you do the same, ( you don't have to ) you could put the new partition there. Here are the commands.
1. 'cd /mnt' ( commands as root, no quotes ).
2. 'mkdir sda5' ( sda5 us just a name, you can call the mount point anything you want ).
Time to mount the formatted the partition.
3. 'mount /dev/sda5 /mnt/sda5' ( press enter, if this works you get no output from the system ). No news is good news.
To help you understand the command, /dev/sda5 is the name the system knows the partition by. This may be different on your system; at least the sda5 may be different. It depends on the partitioning on the drive. If you want to 'see' all the partitions on a drive, you can get a view of them by going to the /dev directory, and do a 'ls sda*' for the first drive, and 'ls sdb*'.
The /mnt/sda5 is the name of the mount point. So, summary, mount - the command, /dev/sdax is the partition the system knows, and /mnt/sdax is the mount point. Thats it.
Also please could you explain what when wrong.
I can not say for sure. You have not provided enough information for me to say. From reading your post, I suspect you have not had much experience with partitioning, formatting, and adding more disk space.
Once the space is formatted and mounted, you probably need to add at least one directory, somewhere to put files. I suggested the dir name of 'download' since you were having problems downloading a large file. 'download' is only a name, you can call it anything that makes sense to you. When you start the download, point your browser to that new directory. The file should go there. If you added 50 gig partition, you should have 50 gigs of space to put files on. In firefox, you can set the place to save files by going to Edit-->Preferences-->General-->Save files to.
also I need to stop my pc from powering off after 5 minutes inactivity.
This sounds like a BIOS setting. Do you know how to change, or at least view the BIOS settings? If not, post the make/model of the system. Its usually a hot key you press, before booting begins.