I work as a Research Assistant whilst doing my PhD and I am salaried, as are the post docs who I work with. I would assume this will be the case for you. At my university post docs start at band 7 which is £29 249 per annum (pre tax) up to £35 938 per annum (pre tax). I would expect you to probably get around 30 grand basically. If you go to http://listentotaxman.com/ you can work out how much you will get after tax etc. University pension schemes are pretty much always worth it, they typically take 7% pre tax and the uni adds the same again. Assuming 7% pension scheme and no UK student loan you will get £1,771 a month on a £30 000 wage. This is £21,252 a year. Without pension it is £1,911 per month and £22,932 per year on a £30,000 wage.
Generally tax is done via the Pay as You Earn (PAYE) system, so the money will be directly transfered to your account each month with all the tax done for you. Make sure they get the tax code right on your pay slip. It should be 810L assuming nothing odd. If they put you on BR (basic rate) then go shout at HR and then claim money back from the tax office. They put people on BR when they can't work out what they should be on. Usually they are just missing a bit of paperwork such as a P60 or P45.
Hope that helps.