Hopefully you have your plan of what to write about (copy) and you'll be trying to design a post to sit alongside it. There are generally agreed guidelines about social media posting - most importantly it should be valuable and/or varied and/or insightful - oh the pressure! Also, you're reminded not to preach about your products and services (but we all know that's why we are there and people aren't daft), so my best advice is to keep your posts varied and don't get hung up on likes - in lots of ways I like to see my social media as a blog of my journey and also a chance to connect with some like minded people. At certain points I do wonder what is the point of what I'm doing and lose my cool but always come back to the fact my real goal is to create social media posts that give value, connect with people and show the real me (but not many selfies as that just freaks me out!), I'm also a little bit self indulgent (e.g. I like challenges and 100 day projects etc). Saying all of that rules are made to be broken - most especially in design - show me a dog post and I'll like it regardless of the above but these are my general design rules to creating a social media post that may help you:
Write your copy aligned to the left as it's easier for people to read (it's harder to read text if it is centred especially if there are more than a couple of lines).
Keep your logo/site tags small and out of the way of your primary message.
If there are a few lines in your post use some weight in headlines to create hierarchy and direct people to read your content in a certain order.
Negative space allows you to create a focal point and lets your design breathe
Write with compatible palettes (using shade) that are easy on the eye (not contrasting as it blows your eyes). This is a link to my post on brand colours and within that it covers the subject of text colours.
Don't write over images as it can be illegible (try set up your type to follow the contour of the image).
I hope this helps! My main social media focus is Instagram - take a look at my instagram posts as examples to help you: www.instagram.com/qumoodesign