So I'm pretty good at writing and describing things...

But I will not be the one GMing a new DH game that I will be joining. The adventure books don't seem to describe certain things except for the major parts of the important keys of the adventure. The person who will be GMing doesn't really know to much about the overall setting or genre...but likes the idea of DH and wants to run the game.

So is there maybe a good link or page on maybe some hints on describing certain objects, places, planets and things within the realm of 40k that could help him?

Here are some guidelines for you to improvise things:

1) Everything that the imperium makes is ornate, covered in religious symbols (scrolls, prayers to the emperor, skulls), and should be ostentatious and grand in either scale or quality.

2) think of a general metaphor for your story (when in doubt, just base it on one of the chaos gods, a kind of xenos, or internal corruption) and try adding it in to your descriptions of things. An adventure based on khorne should focus on the harshness and battle readiness of things, and the anger and red faces of people. An adventure based on orks could focus on lots of things being improvised, or jury rigged, and disorganization among people. Internal corruption should have lots of things looking suspicious, shadowy, or ambiguous/unclear.

3) invite players to describe items occasionally, both to help involve them in the game and to give you a break on making stuff up.

Also, weapons and gear the characters use are not just things to them. The appropriate rites and prayers have to be spoken to appeal to the machine spirit of the object. This tech-religious facet is one of my personal favorites in the WH40K universe

The Imperium is a vast and loosely-connected place and each planet does things more or less their own way, so don't get hung up on the details about how exactly things work because somewhere in the innumerable planets of Man, the thing you just made up is true. The Administratum ensures each planet pays its tithe on time (whatever natural resources the region has, food, people, metals, etc.) and the Ecclesiarchy ensures the Emperor is worshiped, blended with the local superstition if necessary. Beyond that you can pretty much make up whatever you want.

If you need details about a specific thing, Lexicanum is a great resource.

Thanks!