So unfortunately this isn't something with a straight forward answer, as everything has a different value and those values are changing all the time. Generally, you can look up an item in the trading post and in branch sales to see what people are selling that thing for to get an approximation of its current value, generally the lowest selling price will be the most accurate.
Some things are consistently fairly valuable, like applicators for example are usually worth a few GB at least (although there's also a small handful of cheap ones). Many items change value throughout the year as well, items that are currently more accessible because of that months event are going to be less valuable then items that can only be gained during an event a few months prior. So items from say, March, are more valuable right now than items from October, because you cannot currently get new items from the March event, thus making them rare. There are also quite a few items that are so common they have essentially no value. These are decor like Giant Pouched Rat, Charmed Dwarf Lioness, Blue Skimmer, Eager Cub, Fancy Mouse, Scars, Clean Lake, etc. You will commonly find these given away for free or left at the Giving Tree, they are not valuable enough to really trade for anything unfortunately. Food and amusement items/toys pretty much always have some value since they are always needed, but a relatively low value, since they're always readily available every day.
When it comes to lions, lions with mutations are also usually worth at least 1GB, with some mutations like hybrids being worth hundreds of GB. Another easy thing to check is the rarity of their base. Every lion has their "Genetics" listed on their profile, and the last word in the genetics will tell you the rarity. Common, Uncommon, Rare, Special. Special and Rare bases are usually going to be more valuable than Common bases. Another aspect is whether the lions are "Clean" or "Dirty", this refers to whether the lion has had any inbreeding in their lineage, or if their lineage contains the "Big 4" lions, who were 4 lions that in the early days of the game sired a very large amount of cubs, so many that it was hard to not accidentally inbreed lions descended from them, so people preferred to avoid them altogether. Lions without these in their lineage are more valuable than those with. There's other factors of course too, like some specific markings are rare and more valuable like rosettes, and of course, aesthetically pretty lions are worth more than plain looking ones!

If you see a trade you would like to offer on, its most straight forward if there is a buyout price listed, most of the time, the buyout price is how valuable the other person thinks the item is and is how much they would like to get for it. They may or may not actually list items on the trade they are interested in trading for, they may say "LF: (looking for) decor, muties, apps, breeding items, event items" etc. and then you can factor those items into a trade. Sometimes you may want to offer some items, some currency to equal the value of the trade you're interested in.
It's definitely a lot to figure out when you're new, I wouldn't worry about trading too much right away. Hope this helped some though!

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