Welcome Susan! How long have you had your green cheek and what’s their name? You are ahead of many if your gcc doesn’t bite

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It can be difficult to get them to step up when they have places to hide, and they know you can’t see them very well and are hard to reach lol. When mine are on my shoulder or neck and won’t step up, I have reasonably good luck bending over toward something like the back of a chair, a standing perch or something else that I can put my shoulder against and having them step onto that - then I get them to step up onto my finger. Over time, they’ve gotten the idea that it’s faster just to step onto my finger while on my shoulder - I only have one now that still needs the extra step.
Diet - all 4 of my conures came to me as adults on a seed diet and I have transitioned them to pellets and chop (veggies, grains, fruit). It was a bit different for each of them and there’s lots of methods to try - persistence will be key

. I had to try cooked, raw, cut different sizes (tiny to chunks), different combinations..... If your gcc has one item that they like, you can exploit that! Mine liked apples - so I started chopping the apples smaller and smaller (to tiny), then I added a wee bit of a veggie (also chopped up tiny), then tried a different veggie (chopped tiny), and so on. One thing at a time until they become used to seeing different things in the bowl (and they were all ok). Mine are now on a chop that is mostly veggies and grains and only a wee bit of fruit in it - and they all love it. But it takes time for some. Pellets - they don’t necessarily like the first pellet you give them, and you may have to try several. All 4 of mine like different pellets...sigh. Buy small sizes and try a few. Mine eat TOPs, Zupreem fruit, Zupreem natural and Lefaber premium. Harrison’s website has a good guide for switching diet from seeds to pellets, and many here say it has worked for them. Mine were surprisingly easy to switch to pellets. I just played with the pellets outside the cage, made yummy noises and they tried them. Stanley was the hardest to convert and I had to try 4 brands of food before he decided that he would eat Lefaber (of course he didn’t like any of the ones that I already had lol).
Anyhow, just some ideas - I’m sure others will add many great suggestions!