Like others have said OP I think you're overthinking it but it's completely natural, after all you'd hate to cause offense to the bereaved family particularly if it's a fairly formal occasion. But the fact is that so long as you keep uppermost in your mind what is best for the immediate family of the deceased person, you won't go wrong - and if it doesn't affect anyone else then it doesn't matter at all. I've been to way too many funerals over the years, of all kinds and denominations and the things I would say are:
-Like others have said, outfit largely unimportant so long as it's neutral and unobtrusive, and complies with any specific requests made by the family. Even at the most formal/traditional of funerals it hasn't been commonplace to have 100% deep black for all mourners for decades now, at the formal end of the spectrum people will stick to plain/dark-ish colours but you'll see lots of grey, navy, beige etc alongside black, and many many funerals are now much more casual than this. So long as you're not planning on going in fancy dress or gymwear, whatever you have that's smart is fine, no-one expects mourners to buy whole new outfits or anything.
What you do - 95% of the time you get an order of service so just follow along with that and you'll be fine. If in doubt do what everyone else is doing wrt to going into the church, leaving the church, standing up, sitting down etc. Depending on your alignment of religious belief to the specific church you can take communion, sing along with the hymns (unless specified choir only of course!), say the prayers/the congregation responses to the prayer - or you can opt out of all that entirely, or do some parts but not others. It's very common of course that funerals are one of the times people from other faiths and denominations are welcomed into churches and everyone will be understanding that not everyone will participate in everything.
What to say - don't try and come up with something startling or novel if there's a line-up type situation to meet and greet or when you first see your uncle's immediate family. Any variant on 'sorry for your loss', 'my condolences to you and your family', 'thinking of you today' or even just 'love you' is fine. It doesn't matter that they'll have heard it a hundred times already. If you genuinely mean what you say that is worth far more than fancy words. There's usually a lot of hanging around before and after the ceremony and it's fine during this time to make light-ish quiet small talk with your wider family and/or other attendees stood or sat near you - obviously any squealing/gossiping/shouting across the room/loud laughing is probably best saved for afterwards. Sometimes people like to have a little bit of chit chat about the weather or how they knew the deceased beforehand, others would rather just quietly sit/stand and reflect, just be respectful of others' choices (and equally if you don't want to talk to anyone else that's fine too). If you go to the wake/ 'do' afterwards it's more normal to socialise with others a bit, share nice memories of your uncle, it's OK to revert to a more cheerful/normal tone, don't feel obliged to look super-glum and only talk in a sad whisper but again, do what feels right to you, if you're feeling too emotional to do much chatting that's OK to just sit quietly too.
On that note - it's fine to cry and be emotional during the ceremony and afterwards, it's fine to not cry and not be visibly emotional either. None of it has to be judged by some real or imaginary standard of 'correct' grief based on your relationship with the deceased person or their family, there's no such thing as too much or too little. The only thing to avoid is real extremes, so if you are one of the unfortunate people that can't help themselves and do extremely noisy loud sobby-snorty-heaving crying (or even maybe you are a Sistine shrieker) I'd sit at the back and be prepared to quietly slip out and have your cry where you won't draw too much attention, or if the absolute worst happens and you get a fit of the giggles or something else inappropriate, again just pop out and deal with it quietly outside.