It sounds like IMVU is more akin to an MMORPG than SL, which is a virtual world. I've never been to IMVU but having played MMORPGs the "level" nomenclature sounds a lot like MMORPGs.
I would strongly urge you to take some basic building classes offered by many schools for free in SL. (Tipping the instructor is optional, but nice if you benefitted from the class.) There are many schools/classes but off the top of my head I would recommend Builders Brewery and NCI. (NCI is also great place overall for new residents to SL.)
The best way to be notified of classes is to join the groups for the school you want - let's say the two I've mentioned. Then you will receive notices when a class is about to begin. Builder's Brewery also sends out a weekly notice so you can see what classes are offered when and plan accordingly. Alternately or also, you can check the SEARCH function both in world and on the front page of the SL website - also known as your "Dashboard." Search under "Education" and you will see a list of upcoming classes. I know both the schools I mentioned teach beginning building and then other classes to expand on your knowledge.
To specifically answer your questions:
1. A prim is the basic building block in SL. Everything that you see begins with a prim. A prim count is how many prims make up a specific item. For example, let's say I build a sofa - I might use 10 prims to build it. Therefore, the prim count is 10. The reason prim counts are so important (and you will see many items advertised as "low prim") is because there is a limit to how many prims you can have on a given parcel of land, depending on its size.
2. As noted above, prims are the basic elements used to build items in SL. When you first rez one from the build menu - the default is a square box, but "prim torture" is the art of building. 
3. You don't "get prims for a product" you create products *from* prims as noted above.
There is so much more to this - what/where is the build menu, what are the features of the build menu, what are prim limits in size, what are sculpty prims, etc., etc. that, again, taking a class on basic building will be invaluable. I spent one entire year in SL pretty much just taking classes, sometimes 3 a day, but then I'm an education nut...lol. But taking classes is how I learned to build.
A website I would recommend that many people have found useful is:
http://www.mermaiddiaries.com/2006/11/build.html
Natalia Zelmanov began a blog when she first began SL and along the way passed along the tips she learned to build. This site has a TON of great info on it.
Have fun!!
I want to add to the above excellent answers.
If you are making clothes just from textures without any attachments then the prim count is zero. Otherwise you need to rez the attachments and go to edit and the edit box will tell you how many prims each are.
If you are creating things from mesh, be sure to go a step further and not only give the prim count, but the Land Impact number if it is different. (also found in the edit box). Because of the way mesh works in SL, the LI may be different than the actual prim count and its is important that the buyer know this. For instance, someone I know made a vehicle from mesh and the actual prim count was 17 however once the scripted the LI was 237, That means that the buyer would only be able to rez it if they had 237 prims available. Obviously that would be a problem for most people and if they bought it thinking it was only 17 prims they would be very angry customers.
