You enter the sale using the "Stocks, Mutual Funds, Bonds, Other" interview. The "proceeds" information comes from the 1099-B that the broker provides - though 1099-B's aren't due until mid to late February - and, most likely, the "basis" information needs to come from your own records.
Proceeds - Basis = Gain/(Loss)
The capital loss will first be deducted against any offsetting capital gains. If there is capital loss left over after this netting process then $1,500/$3,000 of the loss (depending on filing status) will be deducted against "ordinary" income. If there is still loss left over after using the $1,500/$3,000 amount the remainder will be "carried over" to the next tax year.