Learn Hardcore Japanese: watasi ha nihongo wo rensyuusimasu.

は is the topic‐marker. It tells the listener “as for me…”. If you used が, you’d be marking 私 as the subject in a new or contrastive sense (“It’s me who…”), which isn’t necessary here.

What does the particle を do after 日本語?

を marks the direct object of the verb. Here 日本語 is what you’re practicing, so 日本語を練習します literally means “practice Japanese.”

Why is the verb 練習します placed at the end of the sentence?

Japanese word order is typically Subject > Object > Verb (SOV). So you introduce the topic (私), then the object (日本語), and finally the action (練習します).

Is 練習します present tense, future tense, or something else?

Japanese doesn’t explicitly distinguish present vs. future. 練習します is the non-past polite form, which can mean “(I) practice,” “(I) am going to practice,” or “(I) will practice,” depending on context.

How would I make this sentence casual rather than polite?

Switch the polite ending 〜します to the plain form 〜する.
So it becomes:
私(は)日本語(を)練習する。

Can I omit 私　は and just say 日本語を練習します?

Yes. If the topic or subject is clear from context, Japanese often drops it. 日本語を練習します still means “I practice (or will practice) Japanese.”

What’s the difference between 練習する and 勉強する here?

練習する (to practice) implies doing exercises or drills (speaking drills, conversation practice, etc.).
勉強する (to study) is broader—reading textbooks, learning grammar, vocabulary, etc.

How do I turn this statement into a question (“Do you practice Japanese?”)?

Add the question particle か at the end:
私(は)日本語(を)練習しますか？
Or more naturally, drop 私は and say:
日本語を練習しますか？

Why are there spaces between every word here? Japanese usually doesn’t use spaces, right?

Correct. Native Japanese writing doesn’t use spaces between words. Spaces were added here for teaching clarity—so you can see each element (topic, object, verb) distinctly. Normally you’d write:
私は日本語を練習します。