Yellowish teeth can be due to varied reasons primarily due accumulation of tartar, can be tea/coffee stains. You need to get cleaning done for your teeth and If you are brushing with a soft brush it can  be an addition factor as soft bristles do not clean the teeth properly. Regarding receding gums you need to check with a periodontist as this is not the age where your gums should recede. May be you are brushing vigorously or you have periodontal disease due to which this is happening. It would be helpful if you post a photo of gums or the concerned area of mouth.

Yellowness near gum you are referring to tartar or calculus needs to cleaned with scaling n polishing. And for gum recession you can get periodontal mucogingival surgery. Periodontist is specialist in gum surgery.

Primary treatment for the problem you have mentioned is to get a scaling/ professional cleaning by a dentist. This removes the irritants on the gums and gradually the gums recovers from inflammation. This stops the recession of gums. If you need to correct the existing recession, you will require mucogingival surgery by an expert.