It is easy to get confused when you first begin SEO by the many confusing elements of producing a webpage but two areas are simple to understand in meta tags for ranking purposes on search engines:
a) No mainstream Western search engine uses the meta keywords tag and, as such, there is no need to use this tag for search engines. They don't read it. Sometimes using the tag can be useful for web developers in allowing them to remember what the focus of the webpage is about, but there is no benefit in adding the meta keywords tag when considering Google for example.
b) Google doesn't use the meta description tag for ranking purposes either but tries to find information that matches what the searcher is looking for from the actual web page. This is called a 'snippet' and appears as a description underneath your listing in the search engine. So, if you have no meta description tag, this is what you saw Google displaying as a description - a 'snippet' of information from your page. Personally, I do put in a meta description tag as sometimes Google (and other search engines) use this as a better description than the 'snippet' and some search engines always try and display the meta description - so adding this tag is often useful. Make it a short but attractive statement saying what the web page is about.
So, no, you have done nothing wrong nor made any mistakes from what you have said - but I would add a meta description tag to make any listing that does appear more attractive when a meta description is looked for and used.
Sp again, Welcome to our community and you should find answers to all your questions within the content of this website.
You don't need either the meta description or keyword tags. Don't bother ever using the keywords tag.
Only use the description tag on your most important pages, it's not necessary for every page, especially don't add them if the description will be similar to other pages on your site (e.g. green widgets, red widgets, blue widgets)
... and if you do use a meta description, make it a solid one. The search engines won't say anything about a meta description that's missing entirely. But they will complain if it's too short (50 characters for google, maybe 35 for Bing, no idea what triggers a "too long" complaint) or if it's duplicated across multiple pages.
you cannot specifically submit changes of your meta descriptions to WMT. you must wait until Googlebot crawls the new document and indexes the changes. you can do a "fetch as googlebot" in GWT to speed the process.