How can i fix my crossman 1088?

Sounds like it is leaking, or was maintained using the wrong type of oil, such as 1.5wt airsoft lube - low viscosity oil / penetrating oil just creates leaks. First thing I would try is getting some of the proper lube:

Crosman Pellgunoil

http://www.pyramydair.com/cgi-bin/accessory.pl?acc...

http://www.airgundepot.com/0241.html

Or if you have 20wt~30wt non-detergent motor oil that also will work well (basically the same).

Remove any Co2 cartridge installed, stand it upside-down (grip-up) and put 2 or 3 drops in the Co2 nozzle and let it sit overnight. Then put a drop on the tip of the 12gm Co2 cartridge, install it and fire away.

it might take several rounds of that procedure, it has worked for me on a few old pistols. If not, you can try and rebuild it, get it rebuilt or just get a refurb'd one (it's cheap about $35 http://www.airgundepot.com/crosman-1008sb-reman.ht... ).

The 1088 is one of my favorite plinkers, maybe grab the refurb, the pellgun oil and maybe a speedloader kit (3 clips) budget permitting.

cheers, and good luck