This kind of thing will be tough for anybody to help you with in a forum thread. My suggestion is to get local help.
I would assume by "black screen where you can type" you mean the console? Can you actually login to the shell, or is there an error on startup? Is it taking you into e.g. the grub prompt or is it not booting all the way and bombing into single-user mode or what? We don't even know where you are able to get to.
'fdisk' is the most popular tool in linux to manipulate the partition table, but you haven't even told us what distro you are running or anything, so nobody can even say for certain that will be installed. If you just want to know the steps, you will basically just run fdisk against whatever the name of your disk is - which will also vary based on type of disk and distro. It would normally look something like 'fdisk /dev/sda'. After which you use simple commands like 'p' to print the partition table and 'd' to delete partitions.
Another option is to use 'dd' to just write zeros to every sector.
Another option is to just remove the boot loader so it no longer boots.. this could be grub, it could be lilo, or any number of things, but again you haven't told us.
Not trying to be unhelpful or rude, but there's just way too little info here. It's the equivalent of me just making a forum post saying "guys my car is making a weird noise can someone help me fix it?"