got a 97 sonoma need help with tire sizes! have 18 inch rims 245 45 18's on back dont know what goes on front  

the over all diameter of a 245/45/18 is about 26.8"
You can easily fit that in the front too  

A 225/40/18 will work  

Hope this helps, as it has me in determining tire sizes (apologize for the note-length):
>Tires - Sidewall Markings
Tire sidewall markings might not make sense unless you know the secret handshake. But once you know it, the markings are quite simple and informative, and really quite important. The speed rating is a good example. If your tires are rated for 99mph and you routinely drive faster than that, you could be in for an unpleasant surprise. Modern cars frequently come with governers that limit the top speed via the computer. This is sometimes based on the type of tires the car comes with from the factory, which is of course based on the intended use of the car.
Some rules about how these markings decode and what is required to be put on the tires changed in 1991 and this page reflects that to my knowledge.
A typical sidewall might have something like this:
P215/65R15 87S M+S
Let's break that down:
P   - [optional] Passenger (using the P-metric standard) (LT = light truck, T = temporary spare)
215 - Section Width: sidewall to sidewall width in milimeters. 215mm = 8.5 inches
65  - Aspect Ratio: height of sidewall, bead to tread center. 65% of the width (65% of 215mm = 140mm or 5.5 inches)
R   - Construction: R = radial (D = diagonal, B = bias-belted)
15  - Rim Diamater: diameter of the rim/wheel (15 inches)
87  - Load Index: 87 = 1201 pounds or 545 kilograms (see below)
S   - Speed Rating: S = 112mph or 180km/h (see below)
M+S - Mud & Snow capable
You can figure out the overall tire height using the numbers above. We know that the sidewall is 140mm and there are two "sidewalls" factored into the height, from the road to the bottom of the rim and from the top of the rim to the top of the tire. So we add 140mm and 140mm to get 280mm. Convert that to inches (280/25.4) and we get 11 inches. Now we add that to our rim height (15 inches) and get 26 inches. This number is useful for a couple of reasons. 1) When replacing tires, generally you want to keep the same overall height (to keep the speedometer accurate) but perhaps you want a wider tire. Using the same formula, we can figure out that a 235/60R15 is also 26" tall, but is 20mm wider. 2) Knowing your tire height allows you to do math to figure out RPM at 'x' speed or speed at 'x' RPM, etc...
Here's an example of a slightly more exotic tire.
245/50ZR16 97Y
It decodes like this:
245 - 245mm = 9.6 inches wide
50  - 50% of the width (50% of 245mm = 122mm or 4.8 inches)
ZR  - Z rated radial (Z is the only speed rating that gets listed with the construction type)
16  - 16 inch wheel diamter
97  - load rating 1609 pounds or 730 kilograms (see below)
Y   - speed rating of 300km/hour or 186 mph (see below)
Compared to the 215/65R15, the 245/50ZR16 is a massive performance improvement. It's 30mm wider for better road contact (grip!). It's a much lower profile (50 vs. 65) so it has a shorter sidewall for less deflection in corners (combined with the 16 inch rim). And it's a Z rated tire, capable of much higher speeds. If we were to also list the traction and temperature ratings (see below) the second tire would likely dominate there as well. There are generally two things you give up when going to a performance tire, tread life and snow traction. The performance tires are generally softer for better traction (so they wear faster) and their tread pattern is designed more for dry/wet traction (more rubber on the road, less grooves). But there are tires which do everything well, and they will cost you accordingly.
To give examples of some extremes, let's pick 2 cars and see what tires they come with from the factory.
• a 2001 Dodge Neon comes with P185/65R14's.
o This tire is 7.2 inches wide and has 9.4 inches of total sidewall.
o Cheapest tire in this size: $31 each ($124 for 4)
o Most expensize tire in the size: $81 each ($324 for 4)
• a 2001 Dodge Viper GTS comes with P335/30ZR18's on the rear.
o This tire is 13.1 inches wide and has 7.9 inches of total sidewall.
o Cheapest tire in this size: $382 each ($1,528 for 4)
o Most expensize tire in the size: $507 each ($2,028 for 4)
Treadwear, Temperature and Traction ratings:
• Treadwear: tread life of the tire. this is a number, usually something like 220. Higher number = more tread life. This is set by the manufacturer for comparison purposes only. It does not convert to an actual value.
• Temperature: A, B or C. A is the highest. represents a tire's resistance to heat and its ability to dissipate heat.
• Traction: AA, A, B or C. AA is the highest. represents a tire's ability to stop on wet asphalt and concrete.
The DOT number: For tires manufactured before the year 2000, the last three numbers identify the week and year of manufacture. For example, 459 means the 45th week of 1999. Beginning in 2000, the last four numbers identify the week and year of manufacture. For example, 3500 means the 35th week of 2000. See? Y2K creeps in everywhere. Other characters in the serial number are the tire-maker's coding for tire size, type and manufacturing plant.
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I have a couple "Speed Ratings" & "Load Ratings" along with rim measurement[s] I can email you (or you can get off the NEt). "bondal01@basicisp.net"  Have fun & good luck!