This is often a problem for native speakers of some asian languages, who sometimes confuse L and R. At its best, this yields the classic "President Erect".
L and R are phonetically quite similar. They are both coronal, that is, the tip of the tongue has to touch something in the top of the mouth, and alveolar, meaning that that something is the alveolar ridge, which is right behind the upper teeth.
The difference is that Ls are lateral, while Rs are not. In a lateral consonant, the air flows over one or both sides of the tongue. To turn an R into an L, you have to "break the seal" between your tongue and the roof of the mouth. This can usually be accomplished deliberately in an exaggerated way by saying a continuous R, and shoving or twisting the tongue to one side. The result should sound something like the L sound.
If you are a speaker of a language which possesses the L sound already, you may be able to find which side of your tongue the air flows over by making an L (such as in "lie") and then sharply drawing in a breath. The open side will feel colder.
To confuse matters a little further, L and R each come in two varieties. The word little contains both kinds of L, while the word rare contains both kinds of R. The Ls and Rs I've been talking about are at the hard consonants at the beginning of each of these words. The difference between the soft L and the half-vowel R runs along the same lines, though. Shove the tongue over.