Hi i really need urgent help. I am going to buy a new MTB fork. It's straight steerer tube and threadless. I checked my tires but I have no idea what their size is. It's written 26 × 1.79 and TP 27. There are also lots of option on the website where I am buying the fork from 26er, 27.5er, 29er and 27.5er remote and 29er remote tapered.
if your tires say '26 × 1.79' the you have '26 inch' wheels also know as ISO/ETRTO 599, i.e. the rim diameter is 599mm. the '1.79' is the nominal width of the tire in inches.
Mountain bikes all used to have this size wheel until '29 inch' appeared (ISO/ETRTO 622) and then the compromise '27.5 inch' (ISO/ETRTO 584) - hence the options you see on the website.
To fit your frame you need to make sure the crown to axle length of the new fork is the same as you current fork, otherwise you will raise or lower your headtube and change your head tube angle. If you are buying a fork with the same travel then I'm guessing the fork length will be about the same.
If the steerer is threadless then you’re looking for a 1.125 inch or 1-1/8 inch , or 28.6mm straight steerer (all the same thing).
You also need to match the wheel axle type e.g 9mm Quick Release (most likely) or 15mm x 100 thru axle (less likely).
Lastly you want to match the travel within +/- 20mm. So if your current one is 100mm travel you don’t want to use above 120mm.
New 26 x 1.125 forks seem to be common on many online sites (to my surprise).
You’ll need to cut the steerer to size if you get a new one and possibly install some bearing hardware. This is isn’t an amateur job, find a bike mechanic or at least someone who can cut cleanly and accurately. Remember measure 3 times, cut once. You get exactly one chance.