Get a detector with discrimination
I would suggest you buy something which has discrimination, this lets you know what signals are iron and what aren’t. I started off with a machine which does not have this and was forever digging up nails, broken bits of plough etc.
Also it may be an idea to get a second-hand machine first just in case it turns out that you don’t really like the hobby. You could do worse than look on E-bay. There are always loads of detectors at what seem reasonable prices.
You have to have permission from the landowner or you could have your equipement confiscated, but trust me this is a fun game. It doesn’t always happen at once, you have to perservere, not all fields have finds. Ploughed land I think is the best to detect on but any land will do at the beginning, if there’s a Public Footpath on it even better.
Anyway, to all beginners out there I wish you luck but beware this hobby can be addictive and it’s not just a case of going somewhere where there was say a historical road. From my experience you just never know where the good finds will turn up.
Definitely get a discriminator. Apart from that just go get a machine and have fun.