Welcome to the forum! First, check out the reference thread that SQ linked. It's important to know what is and isn't okay with rats. The reference thread goes more in depth than I do, but here's a small recap.
As it stands right now, Oxbow is the best rat food out there. I found anything that has a lot of seeds or colored food pellets are highly processed or they should be nothing but treats. If you can, try to make homemade treats for your rats, since you'll then know exactly what is going into them. Diet is key! These guys don't live very long unfortunately, so we want to keep them as long as possible.
Veggies, when frozen, are an especially good treat for hot days. My rats don't eat enough veggies in a week (I buy in bulk since it's cheaper) so I almost always buy frozen. Dried stuff tends to be very salty or very sugary. Try to buy organic as frequently as possible. It's expensive, but worth it when it comes to diet.
Just a general rule of thumb is fruits are good, but in low amounts. Too many fruits at once and they could have diarrhea.
When it comes to water, always have a couple of bottles and bowls filled for your rats. Sometimes water bottles don't work. Having multiple of everything leads to less competition. Same goes with food bowls. I feed my rats nightly, unless their food bowls are still full from the night before. I use Oxbow exclusively. My boys like the adult essential diet mixed with the veggie garden stuff.
And please, please, please, don't always take information and accept it without question. Look up things. Research everything. Science is changing near daily, it seems. Critical thinking seems to be a rare trait these days. Some people may not like it, but never be afraid to ask a second opinion. Personally, my rats' health and happiness always trump whether or not someone is offended by my double-checking what they're saying.
