Here is my attempt, wanted to see if this is correct, I'm trying to write a practice presentation for my class on Wednesday I'm a first year Japanese student.
Watashi no Getsuyōbi no gogo goji ni tadaima desu.
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I'm not a professional teacher but please let me try.
I can manage to understand what you mean by your sentence but we (native Japanese speakers) usually use sentences like the following
Watashi wa Getsuyōbi wa gogo goji ni ie ni kaeri masu.
"kaeri masu" is to go back (home = "ie"). We usually omit "watashi wa" and just say
Getsuyōbi wa gogo goji ni ie ni kaeri masu.
Depending on the context, "ie ni" can be omitted.
Getsuyōbi wa gogo goji ni kaeri masu.
Notice that, from none of the above sentences, we can't tell that 5:00pm is the time of arrival or departure. If you want to make clear that it is the arrival time, I think you need to provide the context. For example, if you are arranging weekly Skype meetings or something like that, the important information for the partner of your conversation is the fact that you are at home at that time. In that case, you can say
Getsuyōbi wa gogo goji ni wa ie ni (kaette) imasu,
which means that "On Mondays, I'm home at 5pm at the latest."
I'd suggest the following:
maishū no getsuyōbi wa yūgata goji ni uchi ni tsukimasu.
A few remarks on your attempt:
Since there is no plural in Japanese, I would add 毎週 (maishū; every week) to emphasize that this is a recurring event.
私の (Watashi no) means "my" or "mine". You probably meant to say 私は (Watashi wa) to make "I" the subject of the sentence. However, subjects are frequently omitted in Japanese when they can be guessed from the context. Since the word for "home" has both honorific (お宅; otaku) and humble (家; uchi) forms, the use of the latter makes it clear that you're talking about your home. This makes the subject unnecessary.
午後 (gogo) is not incorrect. But I think in this case, since returning home seems to imply that you've finished your day's work by this point, I would be more inclined to use 夕方 (yūgata; evening).
ただいま (tadaima) is just something people say when they get back home. It's not a verb. To say "I get home", you should use something like 家に着く instead.