Salabarria

Hi im new to this forum and i am also a complete noob when it comes to bikes.

So heres my question, whenever i go to peddle hard my chain constantly slips, im thinking i might need a new chain or the gears are worn out? But im not sure and also ive had the bike for about 5 years now and i always run in dirt trails what parts should i replace on the bike?

And also if someone can give me the link to a website where i can purchases part that would be a big help.

Thanks
(sorry for being a noob)