Writing well comes from practice. While there are some websites that can help, the only way to get better at using any language, which is what you are really asking here, is by making them up and editing what you have created.
If your goal is simple sentences, write several to see if they are expressing your thoughts. Namely:
Susan eats apples on Tuesday.
The elephants laugh at the tourists.
Traffic made me run late.
All the above sentences express a simple idea. These sentences are each called an independent clause.
Simple sentences get boring to write and even more boring to read. Imagine a few pages of sentences that have five words or so. So, you can create a compound sentence. Namely:
Susan eats apples on Tuesday and the elephants laugh at the tourists.
There is no real reason why these two are related, but in a story or an essay, you could have made that clear. Maybe Susan is eating her apples AT THE ZOO and the elephants think that is funny.
You can also create complex sentences. A complex sentence contains an independent clause and a dependent clause. The dependent has info in it that connects it directly to the independent clause. Namely:
Susan eats apples on Tuesday because the elephants laugh at the tourists.
In this sentence, there is now a relationship between what Susan does and what the elephants do. We don’t know WHY there is such a relationship, but that is the subject matter for additional sentences.

Here is a good website that can help you with these matters:
http://grammar.ccc.commnet.edu/grammar/

Writing good sentences become real when you have exposed yourself to good sentence structures. This means that you need to be surrounded with good reading material before you can be acquainted with good sentences. As you read, you get ideas of how good sentences are formed. You are able to visualize what kinds of sentences are in good structure. They are sensible and well thought of.

Writing good sentences become a breeze when you are fond of reading. Through reading, you get plenty of good ideas on grammar and word choice. Your selection of words makes the tone good or bad. Good sentences are found in books, newspapers, and magazines. They act as your models for writing good sentences. The more you practice writing good sentences, the more adept you can become in expressing your ideas.

Writing sentences is an important skill to have. Sentences have to have a purpose. This purpose can be ask a question or tell something. Declarative sentences are the latter and interrogative sentences are the former. In order for a sentence to be complete, it must have a predicate and a subject. A subject can be a pronoun or noun. A predicate has a verb. The predicate describes or is the action that the subject performs. These are simple sentences. Make sure you master the simple sentences first, before moving on to creating more complex sentences. Keep in mind that the more concise you are the better. This method will make you the ideas you would like to convey a lot more clearly to your audience. A final recommendation is to read everything outloud and you will catch more mistakes. Here is a great site for writing sentences:

http://grammar.about.com/od/developingessays/a/quicktips.htm