I'd say if the Tru-Oil has not been opened, and it hasn't hardened in the bottle it should be fine.
Same goes for the other stuff in your refinishing kit.
You say: "...but I don't know anything about stains."
There are 4 basic kinds of stains. Water base, Alcohol base, Oil base and chemical acid or base stains (like aquafortis and lye).
Each has it's merits and demerits.
Water base stains can raise the grain of the wood requireing light resanding after application. It can be thinned with water allowing one to apply several light colored coats until they get the darkness/color they like.
Any water base stain color may be mixed with other colors, or you can apply several coats with each coat using a different color.
Water base stains are compatable with alcohol base stains.
These stains will require some type of oil finish to protect the wood.
Alcohol base stains are much like water base stains except they should be thinned with alcohol if you want to use the multiple application methods as mentioned for the water base stains.
Alcohol stains do not raise the grain, but usually cost quite a bit more (as does the alcohol for thinning them).
Alcohol base stains can be applied over/under water base stains.
They are available at good woodworking stores or at TOTW, MLB, and Dixie Gunworks.
They require an oil base finish to protect the wood.
Oil Base stains do not raise the woods grain but they do add oil protection to the wood. They can be mixed with other oil base stains but are IMO, difficult to thin and use as a multiple application product. There is a definite limit to how much oil the wood can absorb which limits the number of coats that can be applied.
IMO, these are pretty much a "what's in the can is what you get" product.
They are available at Hardware stores and similar places.
Chemical stains modify the wood to develop the color. There is little control over them although with aquafortis carefully applying heat will darken the treated wood. They are usually hazardous to use. They often require neutralization.
Aquafortis is difficult if not impossible to find locally. Lye can be found in the plumbing department of most hardware stores.
In my opnion, if you don't want to invest a lot of money in buying stains consider using Birchwood Casey Walnut Stain.
It isn't going to add any special colors to your stock but it does have three things going for it.
1. It is water base so you can thin it and apply a light coat. Look at the wet surface in the sunlight. If it isn't dark enough add more coats.
2. It isn't very expensive. Some of the Alcohol stains can run over $12 and the thinner is also expensive.
3. Most gun stores and Sporting goods stores carry it.
Everyone has their own likes and dislikes for stock colors. I think with a dark wood like Walnut it is easy to overdo it and end up with a black stock but some people like black.
Good luck to you and let us know how things come out.