No is a complete sentence.
Sometimes we get caught up with the idea that we must have a good reason for saying No to someone and maybe we feel like we can't say No unless we have a good reason. I have a really good reason to say No, because you don't want to! There are of course exceptions to this. My grandmother used to ask for help by saying "who wants to help with the dishes?" I didn't actually want to do dishes but I wanted to help my grandmother, so I did dishes.
There are definitely times to say yes, and there are times to say No. To the person who always asks for help but is unavailable to help you when you need it, just say No. There are people who are just takers and setting boundaries in those relationships is very important to decrease your own anger and or resentment and or feeling less than because they don't offer up more in the relationship.
No does not require an explanation. It can feel like it does, but it doesn't. Someone says, hey, can you help me with this, the answer can be simply No. Now if you need to work up to that, try saying "No I can't that day" or "No I'm not available then" or even "No, I'm sorry I can't." But don't fall into the trap of allowing them to question what you are doing instead. Often a follow up question will be "Oh what are you doing then?" You owe no explanation. You might just say "I'm just not able to help you with your request."
This takes practice so if you aren't perfect at it, no worries, just keep practicing. You'll get there. Healthy relationships are balanced, sometimes Person A helps Person B and sometimes Person B helps person A. It's not necessarily a tit for tat kind of thing, I might help someone emotionally whereas they help me move. People have different things to offer so help might look different for different people.