welcome to WebmasterWorld.
first off (someone's got to tell you) ... you have virtually zero chance of ranking a lettings agency website in the uk ... unless you start learning very quickly indeed or unless you employ somebody who knows what they are doing.
irrespective of the website, the most important thing you need to do is get yourself a google account and a google plus page (for your business, not yourself, don't confuse the two) ... this will enable you to get on google maps, also get yourself on bing maps and mapquest.
go to [
google.com...] and set yourself up with a search console. do the equivalent with bing.
>> I'm having a website made for me which will be all set up, including SEO
there is no such thing as having SEO 'set up', SEO is a longterm process, not something you set up.
>>Do you guys have any recommendations as to what should I be learning first?
depends, do you want to focus on being a letting agent or do you want to develop web skills?
>>Also, what languages are used in website building? How are they integrated if multiple languages are being used?
most 'small' people these days would use php, this is because it is open source and there are no licensing fees, thus servers/hosting is cheaper.
some would say other languages are better, but php is almost ubiquitous, so help is always at hand. also for a beginner, it is very easy to get up and running.
(actually most people probably use word press - which uses php - but that is a whole other question)
>>What's the difference between HTML, php and CSS? (I use lower-case for php because I see it as ".php" at the end of some urls... should it be "PHP"?
HTML ... is a scripting language, that is interpreted by a browser and served as pages to the user - every website is made up of html pages (ignore pedantic exceptions)
CSS ... is a way of styling - eg. laying out the pages in an attractive manner, changes fonts, positions items on the page etc ... a lot of people these days don't use css directly but use a framework (a pre made system of ready created styles), of which bootstrap is the most famous, but there are many others, there are also plenty of good reasons to use a framework and just as many arguments against. bottom line it is much easier to use a framework.
PHP ... is one of many server side 'languages', pages, if they never change can be stored on the server as html (no server side language required), however most pages change continuously, for instance, as a lettings agency you will have new listings all the time, and old listings that are no longer available. most sites with changeable data, store this data in a database. generally, on the server a file queries the database, gets the info it requires, does a bunch of manipulation and then writes an html page 'on the fly' which it sends to the user. any server side language can do this, php is probably the most common.
>>What's HTML5 and what happened to HTML1,2,3 and 4?
who cares, HTML has changed as time has gone on ..the current version is 5, what has gone before is of no consiquence. HTML is the language of the web, a web browser needs to be served a html page in order to show it to the user.
>>What's metadata?
i suspect the person who has built your website has told you that metadata is the keywords and description tag in your html pages.
the keywords tag is a waste of time and has been for many, many years.
the description tag is useful but has no relevance on search engine ranking.
there are other forms of meta data, that are much more advanced, which may be a factor in ranking, such as Microformats, Microdata and RDF's ... but these are concepts several steps ahead of where you are right now... there is plenty of info about themon WebmasterWorld if you want to learn.
>>What else is there that might be useful to know in regards to SEO?
WebmasterWorld is a great resource, i'd start reading, there is a lot of info and pointers here. There is so much stuff to know, i couldn't begin to list it.