How do I send a message from my MacBook Air to a Non-iPhone user? I want to text my friends from my laptop (Apple) and I can't. What should I do?
Assuming you have an iPhone running one of the latest builds and a corresponding Mac (up to date):
Open Messages on your computer.
Sign into the iMessage account that is your current active iMessage Account on your iPhone. You can do this on your Computer's Messages app under the 'Messages' Menu Item > 'Preferences...' > Accounts > '+'
You can find out which Apple ID this is if you're unsure under your iPhone's Settings App > Messages > Send & Receive. (There you will see your Apple ID which you should sign into on your Mac's Messages app and iCloud in System Preferences and any corresponding phone numbers / iMessage Accounts which you can be reached at)
Once you type in your credentials on your Mac, ensure your iPhone and Mac are on the same Wifi.
Make sure that you are signed into the same iCloud account on your computer as on your phone. (On Mac: System Preferences > iCloud)
Back on Messages on your Mac, under the menu: 'Messages' > 'Preferences…', on the side bar with the account settings for your iCloud Associated AppleID account corresponding to your phone you will see "You can be reached for messages at:" under which, if you followed the above 4 steps, you should see your phone number in addition to your appleid email. You may get prompted with an alert from Messages like this:
Do you want to add +1 (555) 555-5555 to iMessage? This phone number was associated with the Apple ID “firstname.lastname@example.org” on “Your iPhone”.
On your iPhone, go to Settings > Messages > Text Message Forwarding and from there select your Mac! That's it! Messages will prompt you with a code to enter on your phone and presto! You've got SMS on your mac through your iPhone.
This will let you bridge your iPhone's SMS capability to Messages on your computer while both devices are on.
You may find that if all that doesn't work, iMessage isn't activated yet on your iPhone. You can check that too under Settings App > Messages> iMessage (and the text under the first toggle will indicate if it is attempting to activate still).
Yes, you can send SMS messages from your Mac to non-Apple phones.
Here is a direct quote from two Support Articles.
Your Mac can receive and send SMS and MMS text messages through your iPhone when you set up text message forwarding. For example, if a friend sends you a text message from a phone other than iPhone, the message appears on your Mac and iPhone in Messages.
SMS messages are green. iMessage messages are blue.
Note: To receive and send SMS and MMS messages on your Mac, your iPhone must have iOS 8.1 or later, and your iPhone and Mac must be signed into iMessage using the same Apple ID. Additionally, you must set up your iPhone.
Set up your iPhone so that you can receive and send SMS and MMS text messages on your Mac. Once you’ve set up your iPhone, if you receive an SMS message on your iPhone, it also appears on your Mac. Plus, when you send a message to a friend who uses a phone other than an iPhone, your message is sent as an SMS message instead.
Note: To receive and send SMS and MMS messages on your Mac, your iPhone must have iOS 8.1 or later, and your iPhone and Mac must be signed into iMessage using the same Apple ID.
On your iPhone, tap Settings > Messages.
Tap Text Message Forwarding.
Enable your Mac in the list of devices.
On your Mac, open Messages.
A six-digit activation code appears.
Enter this code on your iPhone, then tap Allow.
On newer systems, you may not need to do the code part. It just works once you enable the devices in settings.
You can try https://telegram.org which works across Mac, iOS, Android and Windows platforms.