Well, actually I wouldn’t suggest starting out with Fluid simulation…it is a) not THAT good yet and still in development, b) it obviously involves animation which is also not that easy to start with and c) it is so damn time consuming because of the baking times. So, to sum it up, I’d suggest you move on to something else
It is very hard to tell you what you should model - modeling should be fun (at least for us hobbyists) and therefore you should model something, what you WANT to model. For me that was easy - I love mechanical things, stunning cityscapes and “darkness” (uhm, I guess you will know what I mean when you look at my stuff ). Therefore I started with simple machines, tried myself on various techniques and especially tried to plan what I wanted to do - it is really hard to make a complex model (meaning complexity in terms of numbers of meshes) without a plan or at least an image of the object you want to model.
Other people love organic things - characters or something like that. That is pretty hard to start out with (and even for the experienced users a real challenge) so one might want to do something in the middle first. Cars would be an idea here, although they aren’t easy either.
…so, what do I really suggest? Model what you want. If it is a character, okay, then so be it. You shouldn’t be to disappointed when it doesn’t look as you expect it but if you have the basics, you’ll surely learn some general techniques and approaches when you model a character - even if the final outcome isn’t that great.
If you want some satisfaction coming out of the project, too, then it is obviously better to model something simple, as someone suggested, mp3 players or mobile phones are small models, not too complex and with a lot of useful imagery available. But still they provide a good challenge as most of them are made of more or less organic shapes combined with machinery and repetitive detail, which is very common for mechanical modelling. You will most likely dive into both worlds at once.
The next step is not only to model it, but also to make a proper lighting setup and materials as well as textures to make a realistic render (and probably do some postprocessing!). This should not be forgotten when you want to learn CG.
Wish you all the best…whew…this is way too long…