First get a Haynes manual.

If its got hydraulic lifters it could be simply one or more of them has shot it. But inspect the cam first for wear, no point fitting new lifters to a worn cam. While replacing one replace the set. Not as cheap but only needs doing once.

If solid lifters you need to find if one has a larger clearance than specified. Then inpsect the cam for wear especially on that lifter. Shimming the lifters can be aright pain needing a large assortment of shims.

If its a belt drive engine replace it while you are doing the job. Also replace any pulleys.

But since you had to ask I would suggest finding a good garage.

Silly question : there is enough oil in the engine isn't there ? I'd check not only the level, but the condition of the oil. If it hasn't been changed regularly and the incorrect grade used, this will have an effect on the hydraulic lifters as dirty oil can cause the oilways in them to block up.