if i use own SSL certificate which is generated by myself using IIS, is it secure or do i have to buy SSL certifcate from third party.

hi, i have hosted my website which is developed by asp.net and hoseted on IIS server (i have own webhosting provider) . the website is running in http. now i want to change https
so, if i use own SSL certificate which is generated by myself using IIS, is it secure or do i have to buy SSL certifcate from third party.
what will cause if i use SSL certificate which is generated by own? 
Note : i have seperate server where I am hositing my website myself. I am not using third party to host my website. i have own IIS webhostprovider.