I would suggest something like 2 tables:
Transactions table :
TranactionID (make it Autonum)
DateOfTransaction
CompanyName (for the stock)
Stock Symbol
PricePerShare
Shares
BoughtSold
Commision
TotalCost
Everytime you buy or sell it goes into the transaction table.
Dividends table:
TransactionId
Date
Amount
The tansactionID would link the Dividend with the trancaction.