言い方は色々あると思いますので、あくまで一例です。
I’m not good at speaking English.
【訳】英語を話すのは得意ではありません。
My English skills are not that good.
【訳】私の英語力はそんなに良くないです。
I'm not good at ... = 私は〜が苦手です
I'm good at ... = 私は〜が得意です
skill＝「能力」 英語力はヒアリングやらスピーキングやら色々な能力が含まれているだろうということで、複数形で表現することが多いです。
参考になれば幸いです。レッスン頑張ってください！
The two sentences you see provided above are great ways to express to your listener that you are not good at speaking English. In the first sentence you will see the word low. This means something is below what is average. This is word that is appropriate for both formal and informal settings. It would make a great addition to your vocabulary.
If you are saying this phrase to a teacher, I would use: "I am a beginner student.". This would let the teacher know that you are still learning and they will likely then speak more slowly to you. As an online English tutor, I would be happy to have a student tell me this because then I know right away what they believe their English level to be. (Although I find that often students really are better than they think! :) ) I hope this helps!
先生に対してこのフレーズを言うなら、私だったら、
"I am a beginner student."（まだ、初心者です）
を使います。こう言うと先生に、まだ勉強中だと伝わり、たぶんゆっくり話してくれます。
私もオンライン英会話講師の1人ですが、生徒にこんな風に言われたら助かります、生徒が自分の英語力についてどんな風に思っているのかがわかるので（ただ、実際は自分が思っているよりも上手な生徒さんが多いです！）。
参考になれば幸いです！
Maybe, if you want to be extra-polite, you can put a phrase like "excuse me" or "pardon me" or "I'm very sorry" before the sentences above. Of course, there's a difference between "speak English" and "understand English" -- many people understand English reasonably well but then find it difficult to say anything.
特に丁寧に言いたいなら、上記の文の前に、"excuse me" "pardon me" "I'm very sorry" などのフレーズを付けても良いです。
もちろん、英語を話すことと理解することは違います。英語を理解できるけどうまく話せない人はたくさんいます。
"My English skills aren't as strong as my native language skills"
"My English language skills are not so good"
In order to explain that your English spoken language skills are not of a high quality. It would be best to state, "My English skills aren't as strong as my native language skills". This shows that you are still working on your skills. Or, you could simply state "My English language skills are not so good"
英語を話すのが苦手だと伝えたいなら、次のように言うと良いでしょう。
"My English skills aren't as strong as my native language skills"（英語は母国語ほど得意ではありません）
この文では、まだ英語を勉強中であることを表しています。
または、シンプルに次のように言うこともできます。
"My English language skills are not so good"
（英語はあまり得意ではありません）
Hi!
Each of those phrases means the same thing but is said in a different and polite way so that whoever you are speaking with can be prepared to really concentrate and try to understand what you are saying. The first allows you to be confident because it shows you are in the process of learning and you would like them to be patient with you while you try to say something in english. The last one is a bit of slang and is saying I have other skills but english is not my best one at the moment. It is a very nice saying that shows you have other abilities and is intelligent but English is not one of your best abilities yet.
Hope this helps !
日本 
イギリス 
アメリカ合衆国 
カナダ 
