As mdchachi-san, says, ALC is a good resource, as is
Weblio or
a good J-J dictionary (the latter might be intimidating at first, but as you start getting more comfortable reading Japanese, you'll find that dictionaries aimed at natives often offer more nuance and detail than those made for second-language learners.
こんにちは、
みなさん
(みんな is fine as a standalone word, but with さん or さま attached it has to be みなさん
/さま), never みんなさん(/さま)
"center", "core", "middle"等は私がいつも見つける意味です。
I can vaguely understand what you're trying to say here, but it doesn't really flow as natural Japanese.
The は is a bit awkward, as it sounds like you're talking about an established topic, but you're only introducing the meanings here.
Also, the intent of the statement isn't really clear. The reason you're confused is ostensibly not because you "always find" certain meanings, but because the meanings are similar to each other.
This is just an example, but you could say something like:
辞書で調べたら、どれも「center」、「core」、「middle」などと、似たような意味になっています。
When I look them up in a dictionary, they all have similar meanings like "center", "core", and "middle".
どう(or どういうふうに)使い分ければいいのでしょうか?
How should I distinguish between them when I use them?
日本語が下手からなんでもどうぞ誤りを訂正してください。
日本語が下手
だから (or
ですから)
(下手 is an adjectival noun, so the だ/です is necessary for it to be grammatical.)
Also, どうぞ is awkward here, as it makes it sound like you're doing people a favor by asking/allowing them to correct your Japanese.
You'd better off dropping どうぞ and using -て form plus a receiving verb to make it clear you're asking for a favor.
(e.g. 「~訂正 (or 添削) していただけば幸いです。」「~添削していただけると助かります」, or something along those lines.)
Hope this helps!