The easiest/simplest way must be a Dream Induced Lucid Dream (DILD), when you become lucid while you're dreaming by recognising that you're dreaming. To achieve that you can do Reality Checks (RC) during the day and hopefully through force of habit you'll do it while you're dreaming and then you'll realise you're dreaming. There's loads of possible RCs to suit anyone, pick one or two you like. That's the easiest way, but not necessarily the most effective.

You can also try Mnemonic Induction of Lucid Dreams (MILD) which is also easy, but again may not be as effective as other methods. It's basically auto-suggestion, you tell yourself you will recognise you're dreaming.

What's easy for one person might not be as easy for you so have a look at the Tutorials page, choose a method you like and try it out for a while.