Two common sentence errors are comma splices and run on sentences. In both cases, the writer is using no punctuation or not proper punctuation to join 2 independent clauses (sentences). Here are the two most useful options for joining two independent clauses:
1. use a subordinating or coordinating conjunction. An important distinction here is that coordinating conjunctions must have a comma before them, while subordinating conjunctions do not. The coordinating conjunctions can be remembered with acronym FANBOYS: for, and, nor, but, or, yet, so.
2. use a semi-colon.
Here are a few example sentences.
The family was out of milk, so I went to the store to buy more.
The family was out of milk; I went to the store for more.
I had to go to the bank because I needed money. (subordinating conjunction)
I had to go to the bank; I needed money.
There are lots of "right" ways to write the sentence, just be careful and choose the methods that creates the most clarity in your ideas.