I have no knowledge on blogging but as a former print sports journalist who entered the industry with no formal training I figured I would pass on what I was taught or learnt in my time in the industry. Most of it is pretty basic and Billy actually covered a lot already.
1) Write about what you are passionate about. Don't try to force topics that do not interest you. Inevitably they will not be good reads.
2) Write in your own style. This may take you a while to find so do not be afraid to experiment. However, do not try to copy a style you have read and liked if it does not come naturally. This was probably my biggest early mistake in op ed pieces.
3) Write about people. People love to read about people. In the end cards are about people so this should be easy!
4) Write concisely. This includes your sentence structure and your overall post. Don't waffle. Don't use six words when one will do. As you improve as a writer you may develop a more expressive style but generally new writers who head off in to flowery prose are not great to read. I know the internet has unlimited space but you don't need to use it all!
5) Don't bury the lede. This comes from my newspaper background, but, a few "stats" I was told - I take no responsibility for their accuracy! 90% of people will not read further than the first sentence of an article. Of the remaining 10%, 80% of them will not read further than the next paragraph. Get your important information in early.
6) Write accurately. Spell correctly. Learn to use punctuation. Learn correct grammar. Learn what words actually mean. Get your facts right. I am still bad in so many areas but some "professional" writing is terrible.
7) Write, write, write. I did not write enough (might have something to do with the second job I had and working 100 hour weeks) in my time. Write about anything, even if it is not to be published on your blog. Don't burn yourself out, but, be aware that the only way you will improve is by writing more. My early writing makes me cringe. After five years I was merely bad. Almost readable.
8) Get a computer and work area you are comfortable with/in. I have typed all this crap on an IPad - don't do that!
9) Most importantly - enjoy yourself!
(A bonus 10th - don't use too many exclamation points! I am the world's worst offender!)
All the best.