If your signals only become available at market close, you obviously can't execute them in that day's trading session. You have a few options -
Try to execute your trades out of market hours with a willing broker (probably a bad idea, you almost certainly won't get good prices)
Trade in the opening auction the next day.
Trade in the next day's continuous ...
You need an API to interface with a broker (which interfaces to exchanges).
You will have your strategy running, using data from the API, and ultimately the part of your code involved in signal generation will send an API request (in the form of, e.g., “buy 100 shares of AAPL”).
The IB API is quite popular, I would suggest looking through the ...
If you are looking for a quick and easy solution, I have found a combination of Google Sheets + Yahoo Finance URLs to be relatively easy to implement.
Here's an example you could try yourself. Let's say you have a stock ticker, "AAPL" in cell A1.
=INDEX(IMPORTHTML( CONCATENATE("https://finance.yahoo.com/quote/", A2,"?p=",A2,"&.tsrc=fin-srch") ,"table"...
The problem: you do not know the correct ticker that Alpha Vantage can take.
The solution: Use the search endpoint to find the ticker.
Let's say we want to find the correct ticker for Tsingtao Brewery listed in Hong Kong and Shanghai:
Search for the ticker: https://www.alphavantage.co/query?function=SYMBOL_SEARCH&keywords=tsingtao+brewery&apikey=...