You don’t have to do really well, but it generally helps if you want to go down the ‘traditional’ route to becoming an engineer. You have to have good GCSE results in order to do your A levels and get good results there in order to go to university to study engineering.
However, it is possible to go down the route of an apprenticeship and become an engineer that way. I had a friend at secondary school who joined an apprenticeship scheme after he did his GCSEs and now is a qualified engineer designing seats capable of withstanding crashes for helicopters, thus improving the chances of people surviving a crash.
All types of people can become engineers. The 5 of us on this website in the Health Zone all happen to have taken a fairly academic route through to be doing/have done a PhD. But do not fear… there are many other ways of being an engineer check out the Apprentice Zone profiles for how those guys became engineers: https://apprenticej14.imanengineer.org.uk/engineers
The most important GCSEs are passes in Maths, Science and English as these will help you to do calculate things, understand stuff and then write up what you found out.
I don’t think you have to do *really* well at your GCSEs, but good grades in Sciences, Maths and English will help you. Are you doing your GCSEs at the moment?
Although in the health zone we’ve all followed an academic path (A-levels or equivalent, degree, PhD) as Alex and Marcus have said – there are other routes such as apprenticeships, which you can start after GCSEs – you get to learn the skills whilst doing the job, and I’ve got a couple of friends who did this kind of training and it suited them very well. This isn’t any easier than the academic route – just the style of learning whilst you work can really suit some people better. Try the links they’ve suggested and also https://nationalcareersservice.direct.gov.uk/Pages/Home.aspx
Also, it’s worth me saying, I did well at my GCSEs, but not so great at my A-levels – but I’ve still gone on to get a really good degree and PhD 🙂 Persistence and hard work are important too!