I agree with PK, I'd replace the existing bottom bracket with a cartridge type, they're much easier to install and last ages.
You'll need to pull the cranks off, for which you'll need a crank puller, like this:
This is a genuine Shimano crank puller and I recommend it, I have one and it's much better than the unbranded one I had previously.
Like PK says, you can "bodge" the existing bottom bracket out by undoing the lock ring by tapping the lock ring around with a hammer and an old screwdriver, then undoing the inner part with a large adjustable spanner.
You'll need to measure the length of the bottom bracket axle, then get a cartridge bottom bracket with the same length axle. The Shimano UN54 or UN55 aren't expensive smooth running and last for ages:
To fit the cartridhe BB You'll need a tool like this:
Some are like this, which you use with a large spanner, or some have an integral handle, like this: