hi just wondering what offset i need to get 20 inch wheels on my tf sedan fronts go on but trailing arms at the rear get in the way what offset do i need?

On the inside of the rim, mainly on the spokes you should find the offset, it will have a + followed by a number.
OEM offset is +46, my 19's are +38 so they stick out more. Also, width of the rim will be a factor, mine are 8.5inch wide, any wider and they fowl the rear arms. On 20's 8inch might be the max.

thanks im looking at getting some down the track and i know that there will be a catch as i tried to fit 20's a while ago and they didnt go on because of the rear arms