Often, there is a good Wikipedia article available in a language but not in others. Sometimes, a good way to get that information available in other languages is to translate it. You can either take an existing article and simply copy it into a document or the editing window of the target-language Wikipedia, or you can use the content translation tool.
There are a number of other translation tools available to assist you. A few of them are listed at Meta:Babylon/Translations/Translation tools
Many Wikipedias have their own guidelines for how to handle translations. You can find a list here.
Remember that if you translate an article, it's important to show you did so for copyright reasons. Text on Wikimedia sites can be changed and used elsewhere – for example on another language version – but the license we use require that you show where it was taken from. A good first step is to mention that it's a translation and from where in the edit summary field just above the button you click on when you publish the page.
Sources are importantEdit
Sources are incredibly important to Wikipedia. If it's possible, it's good if you can control the sources for the article you translate, but don't let it keep you from translating it. All articles have a talk page. If you haven't been able to check the sources, it could be a good idea to write a short note on the talk page to tell other editors you haven't done so. If the article has no sources, it's probably not worth your time translating it.
Listen to other editorsEdit
Sometimes other editors will disagree with how you have done something. It could be they are mistaken. It could also be they are not. Wikipedia is a collaborative effort and the processes that exist often exist for a reason. Sometimes this is not obvious when you're new. Please still do assume other editors want what's best for the encyclopedia. They should do you the same favour.
Use your userspaceEdit
As soon as you save your page on Wikipedia, others will be able to find it. Because recent changes to the encyclopedia are patrolled to find vandalism and bad edits, they will probably do so almost immediately. If you're not yet done, it's better to save it as a draft in your userspace. To do so, simply write
/Any page name after Special:MyPage. For example, Special:MyPage/Draft.
If you need help, you can ask for it on the wiki where you are translating. There's a list of help pages on different wikis.