If you have disposed of an asset, ie sold shares or fund units, you would enter the net sales proceeds using the T5008. You will have to work out the cost base yourself. Ask your slip provider for help with this.
This needs to be done, as you are reporting a capital gain or los.
If you are receiving a T5008 due only to purchase of shares or units, don't enter anything, as you are not reporting a gain or loss. Retain the T5008 as you will need it to work out your cost base in future when you sell some or all or your investment.