is anyone can help me in designing a stair climbing trolley which has a two real wheel with locking system
Start with a basic specification that answers questions like how much weight it needs to bear, how fast it should move, range of stair angles and heights over which it needs it needs to function, the size of those "real wheels" you mentioned, and so on. This will help you come up with the list of parts you'll need.
Though you can model it as a .sldprt file, and you certainly should start the design with a 'base part' or 'master part' that is a .sldprt file for the "real" parts to reference, it makes more sense to have a complete assembly (.sldasm) in the end because it's much easier to make drawings, BOM's and other things when the "digital" model is exactly representative of the "real" product.
Here's the steps I would take:
step 1. write or obtain a specification
step 2. build a cad 'base model' (.sldprt) that captures the information in the specification in geometry form
step 3. use the model from step 2 as the first feature in all the downstream part models. (google and learn about "top down modeling technique" to understand why, it is a big topic in and of itself)
Step 4. "drag and drop" the downstream part models into sub-assemblies and top level assembly (.sldasm). Because of your work in steps 2 and 3, they all magically fall right into place without the need to "mate" things. (you can always 'float' parts you want to mate a certain way, such as to allow motion simulation)
Step 5. create drawings (.slddrw) if necessary. It's usually necessary.
That should be enough help to get you started and well along the way to designing your stair trolley. As you have specific questions during the design process, you can always come back to this forum because it is a great resource these kind of questions.