If you need a quick tool to merge a couple of songs, Audio Joiner is the tool you need. It’s free, online (no software to install) and easy to use.
You can upload a couple of tracks, for each track you can set the start and end time of the song. Cross fading between the songs is possible but not necessary (you can choose it yourself). You can play the merged song and if you’re happy with it, just ‘join’ button to download your the mp3 of your merged / combined song!