#1
i connect my guitar to pc with jack 3.5 to 6.3 and started play with "guitar rig 5" (program has reduce noise but discontinuous sound) has noise sound

shoud i buy audio interface to connect pc? or do something
#2
Yes, you need an audio interface. The audio input jack on your PC, unless you have a special sound card, is not made for instruments.

I use a Focusrite Scarlett Solo 2nd Gen. It was $99, and it does everything I need it to do.
#3
The cheapest way is a Guitar to USB cable.
Guitars:
Jackson Kelly KE3 - MIJ (Distortion/Jazz)
Jackson DKMGT Dinky (EMG 81/85)
ESP E-II Eclipse Custom (JB/'59)
ESP LTD EC-1001FR (EMG 81/60)
Fender MIM Strat

Amps:
Mesa/Boogie Dual Rectifier Roadster 212
Laney IronHeart IRT-Studio
Peavey Vypyr 30
Peavey ReValver Amp Sims
TOOOO many T.C. Electronic Pedals. . .