In your htdocs > .htaccess file, type the following:
RewriteEngine On
And after that line, add whatever rewrite conditions you like.
You can find a answer to this in this post too: How can i enable mod_rewrite in infinityfree.net - #2 by Ergastolator1
So not to be rude but: my recommendation is before asking a question, check if there is a duplicate of your question, and if there is, check that answer, try the solution, and write here if the solution doesn’t work.
Cheers.