A sentence that expresses a complete thought is also known as an independent clause.
Example: I am going out.
If a sentence starts with a subordinating conjunction, it is also known as a dependent clause, meaning that it cannot express a complete thought.
Example: Because it is New Year's Eve.
In order to make the the dependent clause logical, we need to pair it up with an independent clause.
Example: I am going out because it is New Year's Eve.
Both Because and Since are subordinating conjunctions. A subordinating conjunction introduces a dependent clause, which is then tied to an independent clause.
Examples of Subordinating conjunctions: Since, because, although, while, etc.
Because can be a synonym to Since when Since refers to an explanation or reason for something.
I am going out because it is New Year's Eve.
I am going out since it is New Year's Eve.
About dependent clauses:
Note that when the dependent clause appears after the independent clause, you do not need to use a comma. However, if the dependent clause appears first, then a comma must be used.
Because it is New Year's Eve, I am going out.
Since it is New Year's Eve, I am going out.
Since can also refer to time before now or before a particular time in the past. Often, but not always, the verb in the independent clause of the sentence is in the present perfect or past perfect tense.
I haven't seen some of my university friends since July.
I've been a happier person since becoming a cat lady.
But be careful:
Sometimes, because Since and Because are interchangeable, there can be confusion as to the meaning of the sentence - as to whether you are referring to time or reason.
Since eating dinner, I have felt a little tired.
This lets you wonder. Was I tired because of dinner or just after dinner? If you felt tired because of dinner, then just use Because. If the latter, best to just change the sentence around so that it reads something like this: After dinner I felt tired.