how can i customise the wp template

Hi There

I have my news portal https://www.karavalitimes.com running on joomla platform, which i found quite slow and awful. Now i want to migrate my site to WP.

I have imported the database using WP migrator plugin and thank fully wp is showing all my data in respective categories.. ??

Now my question is how to customise it to give similar look (like joomla). I have downloaded some theme which i liked and installed it.

Now the problem is every time when i post a news it appears on top and old news push down to bottom.. (Like a blog for which WP is developed i guess..)

How can i post particular news in particular section?

Eg: If i upload a news in politics category then it should appear only in that section (instead on the top)– likewise Sports news should go to sports, abc article should go to abc and so on…

Just like a typical news portal where it shows different news in different sections in home page

Is there any documentation available how can i customise the theme as per my requirement?

Thanks a lot in advance

Matheen