I can't tell you how many times I see people of all ages write like this:
The dog nipped at my heals he clamped onto my ankle and wouldn't let go it hurt so much I screamed you want to know why I didn't run well I tried but remember there was a dog clamped to my ankle and it hurt.
That's difficult to read, right? Actually, there are the makings of some exciting sentences in there. You just need to remember The Power of the Period. You need to stamp each independent clause with a period, like this:
The dog nipped at my heals. He clamped onto my ankle and wouldn't let go. It hurt so much I screamed. You want to know why I didn't run? Well, I tried, but remember that there was a dog clamped to my ankle, and it hurt.
Okay, so I added a couple commas and a question mark, too, but it's the periods--our most important marks of punctuation--that do the heavy lifting here. A fun fact: The period is called a "full stop" in the United Kingdom. It may help you to remember that you need to fully stop when you've finished one thought before you move to the next.