I've just started using MobileConnect but I'm confused how Contacts, Keywords, ShortCodes, Messages all relate to one another. Are there any standard guidelines for getting started?
Apart from what C Carswell mentioned above, please consider below points.
Your Contact Key in Mobileconnect Should be same as Subscriber Key used in Email Studio.
If they don't match, it won't be possible to create a journey which has to send emails and SMS to journey contacts. Also only if you match your email studio subscriber key and COntact Key in Mobile Studio, you will get a consolidated view of that contact within Contact Builder.
Whenever a contact changes the mobile number, please make sure you import that contact with Mobile Number into the list with "Priority" variable as 1. this makes sure your contacts updated mobile number is used for sending SMS.
If you are having Salesforce Integration, please use Salesforce AMPScript to update the relevant Contact / Lead, when they send STOP keyword and opt-out from your sms communications.
An example to Getting Started with MobileConnect. Please note that all keywords and campaign information is to be served as an example, and that there are many different ways to utilize MobileConnect.
Create a Keyword such as "marketing". This will be used similarly to an Email Studio "publication list" to keep track of which subscribers are opted in or out. When importing subscribers into MobileConnect, you will be asked to associate with a Keyword, effectively opting these subscribers into the Keyword. This also allows for subscribers to get onto this Keyword using alternative advertising channels (e.g. Text marketing to 12345 to opt-in). After receiving an SMS, subscribers can reply with STOP to opt-out of this Keyword specifically.
Create a Mobile Opt-in message type first, and associate with the "marketing" Keyword. Create the auto-response message for anyone joining the Keyword (as above).
Prepare your list of subscribers to send the message to. The list should contain 3 columns:SubscriberKey,MobileNumber,Locale. SubscriberKey should be the same as other Contacts in Marketing Cloud. This will be used to tie into other imported Email Contacts in Marketing Cloud. MobileNumber should include the international dialing code, but without the "+". Locale should the 2 digit country code of the country you are sending to (such as AU for Australia, or NZ for New Zealand). Save the list as CSV. Note that if subscribers already exist in a Data Extension, it's possible to import them directly from the Data Extension into the Mobile Connect List by using a Contact Builder Import Definition. Make sure the Data Extension field for the Locale is not a Field Type "Locale", as Data Extension Locales (e.g. "en-au") and MobileConnect Locales (e.g. "AU") are two totally different things.
Now import the subscribers into a Standard List in Mobile Connect using "Add Contacts > Import File". Associate these subscribers with the "marketing" Keyword. The header row of your file should map automatically in the import process.
Check that the imported subscriber count is correct. Find your newly imported list here: Manage > Contacts > Lists
Create an Outbound Message Type, and associate the message with your shortcode. You can't link to the Keyword on this message, only the Standard List previously created.
Send out your SMS campaign using the outbound message. Try and keep your SMS message less than 140 characters. If the message is longer than this, you will be charged for each additional message. For instance, if you are sending to 100 subscribers, but your message is 180 characters, 2 messages will be sent to each subscriber, equaling a cost of 200 messages. If you are having trouble with the message limit only being 70 characters, it's best to retype out your message from scratch within the application instead of copy/pasting from another application. Be sure to mention that subscribers can opt-out by replying STOP.
Subscribers will be able to opt out using STOP. This opts them out of the Keyword. If subscribers would like to rejoin the list after being opted out, they would need to SMS "marketing" to the Short Code. They will then receive a confirmation message indicating that they have been re-subscribed. To check subscriber Keyword statuses, go to Manage > Contacts > Lists > [find your subscriber using ContactKey/SubscriberKey] > Membership. Here the contact is associated with various Keyword they have subscribed to, along with the opt-in status. It's also possible to manually update the opt-in status here.
After the message has been sent, subscribers that reply "STOP" will be charged according to their carrier plan. Salesforce is not charging them to opt-out, their carriers are charging them according to their plan. If this is a problem, it might be possible to engage with Salesforce directly to create a Free to End User (FTEU) solution.
Additional campaigns. Sending another outbound message to the same Keyword will work in the same way as a Email Studio Publication List. All previously unsubscribed users will not be sent a message. If a new Keyword is used, the subscriber will be still be sent to. Therefore, it's important to manage Keywords correctly and not to create a new Keyword per campaign.