Hello and welcome.
Did this just start all of a sudden, or has the air temp. been getting less cold over a period of time?
Then this sounds like the freon charge in the system is leaking. When the freon charge gets too low, the compressor will not kick on, and the air temp. will not get cold. A leak would also give the problem you have of the temp. getting less cold over time, since the less freon in the system, the less cold the air will get.
What you will need is to have the A/C system charged and leak checked. Different shops do a leak check different ways, but the most common is to add a dye to the freon when they charge the system, then check with a special light to look for the dye leaking out. It could be as simple as a small O' ring, or a major component like the condenser or evaporator.
The charge for this service depends on the shop, but you can expect to pay 1 hour labor, plus the cost of the freon and dye. Freon and dye would cost approx. $45-$60.
An O' would add $2 for the part, and maybe another 1/2 hour labor.
An evaporator core would add approx. $150 for the part, and an additional approx. 5 hours labor, since the entire dash needs to come out.
A condenser would cost approx. $300, plus an additional approx. 2 hours labor.