I have an approved ESTA but then realised I need a visa J1. I've now applied for the visa, can I just not use the ESTA as it wont cancel? Will it affect anything if I just use the visa (to US) ?
You don't need to "cancel" your ESTA. When you arrive to the US, you'll need to tell the CBP (immigration) officer that you're coming for your exchange studies/work and present the J visa in your passport.
You can use the ESTA if you intend to come to the US for a temporary visit as a tourist or occasional business meetings. Some people have multiple US visas in the same passport and use the appropriate visa for the visit each time, and that's exactly how it is supposed to work. You're fine.
2To emphasise: make sure you present the J visa (and DS-2019) at the border. If you don’t make it clear that you have a J visa, you might be admitted as a visitor, which would mean you wouldn’t be allowed to do your study or internship.– Joe MaltMar 20 at 20:33
To add to @JoeMalt comment, you can immediately check this by looking at the admitted stamp in your passport. The agent should have written J1 under class, not WT or WB (Waiver Tourist/Waiver Business). Mar 21 at 7:03