Open your DAW (digital audio workstation). If you don't have one, try something like FL Studio by Imageline. This DAW's step-by-step method of making music is intuitive and quick. You can get started by downloading the trial version, but if you want to save your compositions, you'll have to buy the full version.
Label your drum tracks. Each drum--your bass drum, snare, hi-hats, etc.--should have a separate track. This takes a little longer and is a tad more work, but it gives more control over the elements in your song, and it will make things easier in the long run.
Make sure the drum tracks' inputs are all set to the virtual drum instrument (your VST drum) you'll be using. Open and activate your drum VST instrument.
Create a four-bar drum phrase for the song by playing it live with a controller or by programming it with the DAW. It doesn't have to be perfect--you can finesse the details later. Once you're finished, copy and paste the bars for so that the song lasts however long you want it to be. Keep in mind, however, that most radio-friendly songs are no longer than four minutes.
For each instrument added, repeat the first four steps. If you're using loops and/or samples, determine the BPM (beats per minute) before inserting them into the song to make sure they match. Many times, the BPM is in the name of the loop. Some DAWs automatically adjust each loop's BPM, so check your DAW's owner's manual.