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Most designs supposed to be done using two strands of embroidery floss. But the floss is usually sold in six strand packaging.
So before threading your needle, you should separate all the 6 strands.
To do it just cut somewhat about 15”-18” (35 - 45 cm) of the thread. Hold it near the top with one hand allowing the bottom to untwist. Now pull each ply upward with the other hand. Carefully - one at a time.
Combine the necessary number of strands (typically two) and save the remaining for the next usage.

Remember, cross-stitch is considered counted embroidery.
So count count count count again and double check your counting. It really helps minimize mistakes.
Gridding save you a lot of time (and your sanity too). Cross stitch is meant to be relaxing. Make your hobby hugely beneficial for your mental and physical health.

The easy to keep from losing your cross-stitch needles, is to glue a magnetic strip to the inside of the box you keep your embroidery floss and other accessories.
It is very important to double check your counting. It is called counted cross stitch, isn't it? If even one missing stitch in important area can destroy the project. So please, check your counting before you start the stitching. It's worth it.

To prevent the floss from tangling, pull it gently but firmly. In case of knot insert tip of the needle downt though the tangle, and then pull it gently, it helps in most cases.

You can cross stitch on different kinds of evenweave fabric. It may be evenweave linen, Aida cloth, waste canvas etc. It is largely a matter of your personal preference.
Just remember: the number of stitches per inch (or per cm) can drastically change the look of your ready project.
Fabric with a higher stitch will produce smaller and finer designs.