How to keep my gears from slipping

Firstly ensure the derailleur is in good condition and not bent so the chain is running perfectly in line with the sprockets. Ensure the stop screws limiting the extent of the travel of the derailleur are set correctly to prevent travel beyond the first and last sprocket.

A badly worn chain could need shortening or replacing in order to provide the correct chain tension or a loose chain could be a matter of a derailleur that hasn't seen the oil can for a while and is not tensioning the chain correctly.
If the bike has been involved in an accident the rear forks could perhaps need some adjustment so a straight edge laid alongside the main chain-wheel coincides approximately with the middle of the rear sprocket cluster.

At this stage it is also wise to ensure the gear cable is in A1 condition, smooth operating and properly oiled with no internal frayed strands causing problems.

If your gear selectors are of the modern style with detents the gear cable needs adjusting so the centre of each detent at the gear selector positions the derailleur precisely below the selected sprocket. Once this is done if everything else mentioned above is ok the gears should now operate smoothly and positively.

If adjustment doesn't work then the problem is likely to be with the basic requirements above.
Good luck!

Need more info to answer accurately. Does it shift back to higher gears (harder to pedal) or does is sometimes try to shift just one gear up or down (catching on the next gear). If the former, then the shift lever pivot is loose. If the latter then the index adjustment is off. It is NOT due to chain length/tension nor to limit adjustments.

Adjusting the indexing is very easy, and usually requires no tools. Look at the index adjustment portion on the following two links to parktool.com and sheldonbrown.com (best sources of repair help I have found).
Rear Derailleur Adjustment
Derailer Adjustment
If you have a bike co-op in town they can be of much help

I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881