If you have stored telescope for some time, but now wish to bring it back into service, there are almost certainly a few things that you will need to check and do before you use it.
How clean it is will depend on where the telescope was stored and how it was wrapped and sealed. If it was stored in a garage or an outbuilding, there is a good chance that the reflector may have been invaded by insects and spiders. If this is the case, it may be necessary to remove the mirror and its cell and clean out the inside of the tube with a soft brush, being sure to remove all visible debris. The mirror itself may need cleaning provided that the surface coating has not degraded. You can do this by carefully washing the mirror with de-ionised water. If you own a refractor, it is much less likely that insects will have found their way into the tube, although not impossible. Again you will need to remove the lens and cell from the tube and clean it out thoroughly.
On re-assembly, check that the collimation is good and adjust it if necessary. Make sure that mechanical parts of the telescope and mount, such as the focuser and bearings on the mount, are clean and regrease them if necessary. Clean off any rust or corrosion first. If your mount uses a drive, you should check this thoroughly too. For example, make sure that forgotten batteries haven't leaked in their compartments. After all, this has been completed, you should be ready to start observing again.
Cleaning Telescope Mirror
I had stored my Newtonian in the wardrobe, mirror facing downwards, for the past 4 years. Even so, the surface was covered with dust as you can see in the picture below.
Having blown much of the loose dust off with a pressurised can of air, I thought it best to do the whole thing properly, so I rinsed it under the tap, then submerged it in warm soapy water. I used normal washing up liquid. I very gently wiped the surface with a clean foam applicator pad, one that I would normally use for polishing my car. After another rinse with fresh water, I let it dry.
After drying out, there were several dirty marks where the water had left marks as it dried. I sprayed the surface with my camera lens cleaning gel, and wiped it clean with the lens cloth. This brought the mirror back to a flawless condition.
The secondary mirror was nowhere near as badly dusted over, so I was able to just wipe it over with the lens cloth. The rest of the tube was clean.
After I had reinserted the primary mirror into the tube, I performed a quick collimation (which was way out) before taking the telescope outside ready for the night's observation. Once the telescope had cooled down to outside temperature, I did a more accurate collimation.