The following is written in iams. I suggest you read this node aloud to best discern the rhythm.
There are several types of beats that make up basic rhythm, rhythms used since forms were first transported here to English from its brother languages. French, Italian lines inspired most of Middle English poesy.
If you want to write a rhythmic line you'd use a form with iams, trochees, dactyls, anapests, or even spondees. Iambic lines are oldest, though; they've been in use since the beginning to create heroic couplets, then some sonnets, now blank verse.
If you have a hard time telling when this sort of rhythm is in use, then it's best to draw a diagram of each line that you're reading, where you mark each syllable and accent. If the accents fall on every other syllable then it's likely that you're reading bits of pentameter, iambic.
To create the form yourself it's best to have a bit of background reading works out loud that use the form, like Chaucer's Canterbury Tales (in Middle English), or some of Shakespeare's sonnets. Don't despair; crafting rhythm and a meter takes some difficulty doing.