To get started - pick two songs that sound very familiar to you. Check that they are the same bpm (or very close) and try to mix them together.
You'll find that many songs have parts that are ideal for mixing in the next song - but NEVER try to mix over vocals - it never sounds good! Look for the bridge or instrumental break and experiment at that point. Of course, dance music is probably the best type of music to mix - in my experience, house music is best (but I love house, so I would say that!).
When starting out as a mix DJ, I found that songs that were in-between 125-130bpm were the easist to beatmatch. Once you've mastered that range you can be creative and do slower or faster mixes that will drive your audience wild!
The best advice I can give though, is start with the music you know really well, and practice lots.
I wish you the best of luck...
Kevin
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